diff --git a/mailpoet/RoboFile.php b/mailpoet/RoboFile.php
index 25a5244502..b0f6d363c1 100644
--- a/mailpoet/RoboFile.php
+++ b/mailpoet/RoboFile.php
@@ -1383,7 +1383,7 @@ class RoboFile extends \Robo\Tasks {
     $this->say("Release '$version[name]' info was published on Slack.");
   }
 
-  public function releaseRerunCircleWorkflow(string $project = null) {
+  public function releaseRerunCircleWorkflow(?string $project = null) {
     $circleciController = $this->createCircleCiController();
     $result = $circleciController->rerunLatestWorkflow($project);
     // Sometimes can be useful to know which Circle project workflow was restarted
diff --git a/mailpoet/lib/API/JSON/ResponseBuilders/NewslettersResponseBuilder.php b/mailpoet/lib/API/JSON/ResponseBuilders/NewslettersResponseBuilder.php
index 98b45207b2..b763d83c1a 100644
--- a/mailpoet/lib/API/JSON/ResponseBuilders/NewslettersResponseBuilder.php
+++ b/mailpoet/lib/API/JSON/ResponseBuilders/NewslettersResponseBuilder.php
@@ -158,7 +158,7 @@ class NewslettersResponseBuilder {
    * @param SendingQueueEntity|null $latestQueue
    * @return array<string, mixed>
    */
-  private function buildListingItem(NewsletterEntity $newsletter, NewsletterStatistics $statistics = null, SendingQueueEntity $latestQueue = null): array {
+  private function buildListingItem(NewsletterEntity $newsletter, ?NewsletterStatistics $statistics = null, ?SendingQueueEntity $latestQueue = null): array {
     $couponBlockLogs = array_map(function ($item) {
       return "Coupon block: $item";
     }, $this->logRepository->getRawMessagesForNewsletter($newsletter, LoggerFactory::TOPIC_COUPONS));
diff --git a/mailpoet/lib/API/REST/Response.php b/mailpoet/lib/API/REST/Response.php
index 09c7e01bde..bc51e10fb8 100644
--- a/mailpoet/lib/API/REST/Response.php
+++ b/mailpoet/lib/API/REST/Response.php
@@ -6,7 +6,7 @@ use WP_REST_Response;
 
 class Response extends WP_REST_Response {
   public function __construct(
-    array $data = null,
+    ?array $data = null,
     int $status = 200
   ) {
     parent::__construct(['data' => $data], $status);
diff --git a/mailpoet/lib/AdminPages/Pages/Help.php b/mailpoet/lib/AdminPages/Pages/Help.php
index 936fa9b593..b5353c150f 100644
--- a/mailpoet/lib/AdminPages/Pages/Help.php
+++ b/mailpoet/lib/AdminPages/Pages/Help.php
@@ -107,7 +107,7 @@ class Help {
     return $actionSchedulerData;
   }
 
-  private function getLatestActionSchedulerActionDate(string $hook, string $status = null): ?string {
+  private function getLatestActionSchedulerActionDate(string $hook, ?string $status = null): ?string {
     $query = [
       'per_page' => 1,
       'order' => 'DESC',
diff --git a/mailpoet/lib/AutomaticEmails/WooCommerce/Events/FirstPurchase.php b/mailpoet/lib/AutomaticEmails/WooCommerce/Events/FirstPurchase.php
index 6c51aae16b..4433c8b8de 100644
--- a/mailpoet/lib/AutomaticEmails/WooCommerce/Events/FirstPurchase.php
+++ b/mailpoet/lib/AutomaticEmails/WooCommerce/Events/FirstPurchase.php
@@ -36,7 +36,7 @@ class FirstPurchase {
   private $subscribersRepository;
 
   public function __construct(
-    WCHelper $helper = null
+    ?WCHelper $helper = null
   ) {
     if ($helper === null) {
       $helper = ContainerWrapper::getInstance()->get(WCHelper::class);
diff --git a/mailpoet/lib/AutomaticEmails/WooCommerce/Events/PurchasedInCategory.php b/mailpoet/lib/AutomaticEmails/WooCommerce/Events/PurchasedInCategory.php
index c23601582c..79d3c1ebae 100644
--- a/mailpoet/lib/AutomaticEmails/WooCommerce/Events/PurchasedInCategory.php
+++ b/mailpoet/lib/AutomaticEmails/WooCommerce/Events/PurchasedInCategory.php
@@ -34,7 +34,7 @@ class PurchasedInCategory {
   private $subscribersRepository;
 
   public function __construct(
-    WCHelper $woocommerceHelper = null
+    ?WCHelper $woocommerceHelper = null
   ) {
     if ($woocommerceHelper === null) {
       $woocommerceHelper = ContainerWrapper::getInstance()->get(WCHelper::class);
diff --git a/mailpoet/lib/AutomaticEmails/WooCommerce/Events/PurchasedProduct.php b/mailpoet/lib/AutomaticEmails/WooCommerce/Events/PurchasedProduct.php
index 0156954f81..29694fb181 100644
--- a/mailpoet/lib/AutomaticEmails/WooCommerce/Events/PurchasedProduct.php
+++ b/mailpoet/lib/AutomaticEmails/WooCommerce/Events/PurchasedProduct.php
@@ -35,7 +35,7 @@ class PurchasedProduct {
   private $subscribersRepository;
 
   public function __construct(
-    WCHelper $helper = null
+    ?WCHelper $helper = null
   ) {
     if ($helper === null) {
       $helper = ContainerWrapper::getInstance()->get(WCHelper::class);
diff --git a/mailpoet/lib/Automation/Engine/Control/StepRunController.php b/mailpoet/lib/Automation/Engine/Control/StepRunController.php
index 05521351d6..b43f1946bd 100644
--- a/mailpoet/lib/Automation/Engine/Control/StepRunController.php
+++ b/mailpoet/lib/Automation/Engine/Control/StepRunController.php
@@ -25,15 +25,15 @@ class StepRunController {
     $this->stepRunLogger = $stepRunLogger;
   }
 
-  public function scheduleProgress(int $timestamp = null): int {
+  public function scheduleProgress(?int $timestamp = null): int {
     return $this->stepScheduler->scheduleProgress($this->stepRunArgs, $timestamp);
   }
 
-  public function scheduleNextStep(int $timestamp = null): int {
+  public function scheduleNextStep(?int $timestamp = null): int {
     return $this->stepScheduler->scheduleNextStep($this->stepRunArgs, $timestamp);
   }
 
-  public function scheduleNextStepByIndex(int $nextStepIndex, int $timestamp = null): int {
+  public function scheduleNextStepByIndex(int $nextStepIndex, ?int $timestamp = null): int {
     return $this->stepScheduler->scheduleNextStepByIndex($this->stepRunArgs, $nextStepIndex, $timestamp);
   }
 
diff --git a/mailpoet/lib/Automation/Engine/Control/StepRunLogger.php b/mailpoet/lib/Automation/Engine/Control/StepRunLogger.php
index d727431439..0006907339 100644
--- a/mailpoet/lib/Automation/Engine/Control/StepRunLogger.php
+++ b/mailpoet/lib/Automation/Engine/Control/StepRunLogger.php
@@ -42,7 +42,7 @@ class StepRunLogger {
     string $stepId,
     string $stepType,
     int $runNumber,
-    bool $isWpDebug = null
+    ?bool $isWpDebug = null
   ) {
     $this->automationRunLogStorage = $automationRunLogStorage;
     $this->hooks = $hooks;
diff --git a/mailpoet/lib/Automation/Engine/Control/StepScheduler.php b/mailpoet/lib/Automation/Engine/Control/StepScheduler.php
index 94f3bf48c0..caf0c0627e 100644
--- a/mailpoet/lib/Automation/Engine/Control/StepScheduler.php
+++ b/mailpoet/lib/Automation/Engine/Control/StepScheduler.php
@@ -23,13 +23,13 @@ class StepScheduler {
     $this->automationRunStorage = $automationRunStorage;
   }
 
-  public function scheduleProgress(StepRunArgs $args, int $timestamp = null): int {
+  public function scheduleProgress(StepRunArgs $args, ?int $timestamp = null): int {
     $runId = $args->getAutomationRun()->getId();
     $data = $this->getActionData($runId, $args->getStep()->getId(), $args->getRunNumber() + 1);
     return $this->scheduleStepAction($data, $timestamp);
   }
 
-  public function scheduleNextStep(StepRunArgs $args, int $timestamp = null): int {
+  public function scheduleNextStep(StepRunArgs $args, ?int $timestamp = null): int {
     $step = $args->getStep();
     $nextSteps = $step->getNextSteps();
 
@@ -46,7 +46,7 @@ class StepScheduler {
     return $this->scheduleNextStepByIndex($args, 0, $timestamp);
   }
 
-  public function scheduleNextStepByIndex(StepRunArgs $args, int $nextStepIndex, int $timestamp = null): int {
+  public function scheduleNextStepByIndex(StepRunArgs $args, int $nextStepIndex, ?int $timestamp = null): int {
     $step = $args->getStep();
     $nextStep = $step->getNextSteps()[$nextStepIndex] ?? null;
     if (!$nextStep) {
@@ -95,7 +95,7 @@ class StepScheduler {
     return $this->hasScheduledNextStep($args) || $this->hasScheduledProgress($args);
   }
 
-  private function scheduleStepAction(array $data, int $timestamp = null): int {
+  private function scheduleStepAction(array $data, ?int $timestamp = null): int {
     return $timestamp === null
       ? $this->actionScheduler->enqueue(Hooks::AUTOMATION_STEP, $data)
       : $this->actionScheduler->schedule($timestamp, Hooks::AUTOMATION_STEP, $data);
diff --git a/mailpoet/lib/Automation/Engine/Data/Automation.php b/mailpoet/lib/Automation/Engine/Data/Automation.php
index bd34c2c36c..cc59a3b7a6 100644
--- a/mailpoet/lib/Automation/Engine/Data/Automation.php
+++ b/mailpoet/lib/Automation/Engine/Data/Automation.php
@@ -53,8 +53,8 @@ class Automation {
     string $name,
     array $steps,
     \WP_User $author,
-    int $id = null,
-    int $versionId = null
+    ?int $id = null,
+    ?int $versionId = null
   ) {
     $this->name = $name;
     $this->steps = $steps;
diff --git a/mailpoet/lib/Automation/Engine/Data/AutomationRun.php b/mailpoet/lib/Automation/Engine/Data/AutomationRun.php
index 972e2d5be7..a6dcea1071 100644
--- a/mailpoet/lib/Automation/Engine/Data/AutomationRun.php
+++ b/mailpoet/lib/Automation/Engine/Data/AutomationRun.php
@@ -42,7 +42,7 @@ class AutomationRun {
     int $versionId,
     string $triggerKey,
     array $subjects,
-    int $id = null
+    ?int $id = null
   ) {
     $this->automationId = $automationId;
     $this->versionId = $versionId;
@@ -91,7 +91,7 @@ class AutomationRun {
   }
 
   /** @return Subject[] */
-  public function getSubjects(string $key = null): array {
+  public function getSubjects(?string $key = null): array {
     if ($key) {
       return array_values(
         array_filter($this->subjects, function (Subject $subject) use ($key) {
diff --git a/mailpoet/lib/Automation/Engine/Data/AutomationRunLog.php b/mailpoet/lib/Automation/Engine/Data/AutomationRunLog.php
index 358a73c7ea..cebc3d226b 100644
--- a/mailpoet/lib/Automation/Engine/Data/AutomationRunLog.php
+++ b/mailpoet/lib/Automation/Engine/Data/AutomationRunLog.php
@@ -60,7 +60,7 @@ class AutomationRunLog {
     int $automationRunId,
     string $stepId,
     string $stepType,
-    int $id = null
+    ?int $id = null
   ) {
     $this->automationRunId = $automationRunId;
     $this->stepId = $stepId;
diff --git a/mailpoet/lib/Automation/Engine/Data/Step.php b/mailpoet/lib/Automation/Engine/Data/Step.php
index e7c04e0664..6962e8efe3 100644
--- a/mailpoet/lib/Automation/Engine/Data/Step.php
+++ b/mailpoet/lib/Automation/Engine/Data/Step.php
@@ -35,7 +35,7 @@ class Step {
     string $key,
     array $args,
     array $nextSteps,
-    Filters $filters = null
+    ?Filters $filters = null
   ) {
     $this->id = $id;
     $this->type = $type;
diff --git a/mailpoet/lib/Automation/Engine/Exceptions/Exception.php b/mailpoet/lib/Automation/Engine/Exceptions/Exception.php
index 10a2cf5dae..161a72806b 100644
--- a/mailpoet/lib/Automation/Engine/Exceptions/Exception.php
+++ b/mailpoet/lib/Automation/Engine/Exceptions/Exception.php
@@ -20,16 +20,16 @@ abstract class Exception extends PhpException implements RestException {
   protected $errors = [];
 
   final public function __construct(
-    string $message = null,
-    string $errorCode = null,
-    Throwable $previous = null
+    ?string $message = null,
+    ?string $errorCode = null,
+    ?Throwable $previous = null
   ) {
     parent::__construct($message ?? __('Unknown error.', 'mailpoet'), 0, $previous);
     $this->errorCode = $errorCode ?? 'mailpoet_automation_unknown_error';
   }
 
   /** @return static */
-  public static function create(Throwable $previous = null) {
+  public static function create(?Throwable $previous = null) {
     return new static(null, null, $previous);
   }
 
diff --git a/mailpoet/lib/Automation/Engine/Mappers/AutomationMapper.php b/mailpoet/lib/Automation/Engine/Mappers/AutomationMapper.php
index 0ffbbbb9e7..d15879b45c 100644
--- a/mailpoet/lib/Automation/Engine/Mappers/AutomationMapper.php
+++ b/mailpoet/lib/Automation/Engine/Mappers/AutomationMapper.php
@@ -19,7 +19,7 @@ class AutomationMapper {
     $this->statisticsStorage = $statisticsStorage;
   }
 
-  public function buildAutomation(Automation $automation, AutomationStatistics $statistics = null): array {
+  public function buildAutomation(Automation $automation, ?AutomationStatistics $statistics = null): array {
 
     return [
       'id' => $automation->getId(),
diff --git a/mailpoet/lib/Automation/Engine/Registry.php b/mailpoet/lib/Automation/Engine/Registry.php
index b37f4babde..ad5b242d98 100644
--- a/mailpoet/lib/Automation/Engine/Registry.php
+++ b/mailpoet/lib/Automation/Engine/Registry.php
@@ -98,7 +98,7 @@ class Registry {
   }
 
   /** @return array<string, AutomationTemplate> */
-  public function getTemplates(string $category = null): array {
+  public function getTemplates(?string $category = null): array {
     return $category
       ? array_filter(
         $this->templates,
@@ -254,7 +254,7 @@ class Registry {
     $this->wordPress->addAction(Hooks::AUTOMATION_BEFORE_SAVE, $callback, $priority);
   }
 
-  public function onBeforeAutomationStepSave(callable $callback, string $key = null, int $priority = 10): void {
+  public function onBeforeAutomationStepSave(callable $callback, ?string $key = null, int $priority = 10): void {
     $keyPart = $key ? "/key=$key" : '';
     $this->wordPress->addAction(Hooks::AUTOMATION_STEP_BEFORE_SAVE . $keyPart, $callback, $priority, 2);
   }
diff --git a/mailpoet/lib/Automation/Engine/Storage/AutomationRunLogStorage.php b/mailpoet/lib/Automation/Engine/Storage/AutomationRunLogStorage.php
index 13a49fc03f..7b65616cd5 100644
--- a/mailpoet/lib/Automation/Engine/Storage/AutomationRunLogStorage.php
+++ b/mailpoet/lib/Automation/Engine/Storage/AutomationRunLogStorage.php
@@ -32,7 +32,7 @@ class AutomationRunLogStorage {
     }
   }
 
-  public function getAutomationRunStatisticsForAutomationInTimeFrame(int $automationId, string $status, \DateTimeImmutable $after, \DateTimeImmutable $before, int $versionId = null): array {
+  public function getAutomationRunStatisticsForAutomationInTimeFrame(int $automationId, string $status, \DateTimeImmutable $after, \DateTimeImmutable $before, ?int $versionId = null): array {
     global $wpdb;
     $andWhere = $versionId ? 'AND run.version_id = %d' : '';
     $results = $wpdb->get_results(
diff --git a/mailpoet/lib/Automation/Engine/Storage/AutomationRunStorage.php b/mailpoet/lib/Automation/Engine/Storage/AutomationRunStorage.php
index ecc8d12ebd..352a659bb0 100644
--- a/mailpoet/lib/Automation/Engine/Storage/AutomationRunStorage.php
+++ b/mailpoet/lib/Automation/Engine/Storage/AutomationRunStorage.php
@@ -218,7 +218,7 @@ class AutomationRunStorage {
     }
   }
 
-  public function getAutomationStepStatisticForTimeFrame(int $automationId, string $status, \DateTimeImmutable $after, \DateTimeImmutable $before, int $versionId = null): array {
+  public function getAutomationStepStatisticForTimeFrame(int $automationId, string $status, \DateTimeImmutable $after, \DateTimeImmutable $before, ?int $versionId = null): array {
     global $wpdb;
     $andWhere = $versionId ? 'AND version_id = %d' : '';
     $result = $wpdb->get_results(
diff --git a/mailpoet/lib/Automation/Engine/Storage/AutomationStatisticsStorage.php b/mailpoet/lib/Automation/Engine/Storage/AutomationStatisticsStorage.php
index d20fc59d50..1b34624133 100644
--- a/mailpoet/lib/Automation/Engine/Storage/AutomationStatisticsStorage.php
+++ b/mailpoet/lib/Automation/Engine/Storage/AutomationStatisticsStorage.php
@@ -39,7 +39,7 @@ class AutomationStatisticsStorage {
     return $statistics;
   }
 
-  public function getAutomationStats(int $automationId, int $versionId = null, \DateTimeImmutable $after = null, \DateTimeImmutable $before = null): AutomationStatistics {
+  public function getAutomationStats(int $automationId, ?int $versionId = null, ?\DateTimeImmutable $after = null, ?\DateTimeImmutable $before = null): AutomationStatistics {
     $data = $this->getStatistics([$automationId], $versionId, $after, $before);
     return new AutomationStatistics(
       $automationId,
@@ -53,7 +53,7 @@ class AutomationStatisticsStorage {
    * @param int[] $automationIds
    * @return array<int, array{id: int, total: int, running: int}>
    */
-  private function getStatistics(array $automationIds, int $versionId = null, \DateTimeImmutable $after = null, \DateTimeImmutable $before = null): array {
+  private function getStatistics(array $automationIds, ?int $versionId = null, ?\DateTimeImmutable $after = null, ?\DateTimeImmutable $before = null): array {
     global $wpdb;
     $totalSubquery = $this->getStatsQuery($automationIds, $versionId, $after, $before);
     $runningSubquery = $this->getStatsQuery($automationIds, $versionId, $after, $before, AutomationRun::STATUS_RUNNING);
@@ -71,7 +71,7 @@ class AutomationStatisticsStorage {
     return array_combine(array_column($results, 'id'), $results) ?: [];
   }
 
-  private function getStatsQuery(array $automationIds, int $versionId = null, \DateTimeImmutable $after = null, \DateTimeImmutable $before = null, string $status = null): string {
+  private function getStatsQuery(array $automationIds, ?int $versionId = null, ?\DateTimeImmutable $after = null, ?\DateTimeImmutable $before = null, ?string $status = null): string {
     global $wpdb;
 
     $versionCondition = $versionId ? 'AND version_id = %d' : '';
diff --git a/mailpoet/lib/Automation/Engine/Storage/AutomationStorage.php b/mailpoet/lib/Automation/Engine/Storage/AutomationStorage.php
index 3a478c1242..a7b098529a 100644
--- a/mailpoet/lib/Automation/Engine/Storage/AutomationStorage.php
+++ b/mailpoet/lib/Automation/Engine/Storage/AutomationStorage.php
@@ -126,7 +126,7 @@ class AutomationStorage {
     ) : [];
   }
 
-  public function getAutomation(int $automationId, int $versionId = null): ?Automation {
+  public function getAutomation(int $automationId, ?int $versionId = null): ?Automation {
     global $wpdb;
 
     if ($versionId) {
@@ -153,7 +153,7 @@ class AutomationStorage {
   }
 
   /** @return Automation[] */
-  public function getAutomations(array $status = null): array {
+  public function getAutomations(?array $status = null): array {
     global $wpdb;
 
     $statusFilter = $status ? 'AND a.status IN (' . implode(',', array_fill(0, count($status), '%s')) . ')' : '';
@@ -184,7 +184,7 @@ class AutomationStorage {
   }
 
   /** @return int[] */
-  public function getAutomationIdsBySubject(Subject $subject, array $runStatus = null, int $inTheLastSeconds = null): array {
+  public function getAutomationIdsBySubject(Subject $subject, ?array $runStatus = null, ?int $inTheLastSeconds = null): array {
     global $wpdb;
 
     $statusFilter = $runStatus ? 'AND r.status IN (' . implode(',', array_fill(0, count($runStatus), '%s')) . ')' : '';
diff --git a/mailpoet/lib/Automation/Engine/WordPress.php b/mailpoet/lib/Automation/Engine/WordPress.php
index a9abefc60b..ed52c4fa39 100644
--- a/mailpoet/lib/Automation/Engine/WordPress.php
+++ b/mailpoet/lib/Automation/Engine/WordPress.php
@@ -68,7 +68,7 @@ class WordPress {
   }
 
   /** @return WP_Post[]|int[] */
-  public function getPosts(array $args = null): array {
+  public function getPosts(?array $args = null): array {
     return get_posts($args);
   }
 
diff --git a/mailpoet/lib/Automation/Integrations/MailPoet/Actions/SendEmailAction.php b/mailpoet/lib/Automation/Integrations/MailPoet/Actions/SendEmailAction.php
index 5596f7c12b..f6505c3bd2 100644
--- a/mailpoet/lib/Automation/Integrations/MailPoet/Actions/SendEmailAction.php
+++ b/mailpoet/lib/Automation/Integrations/MailPoet/Actions/SendEmailAction.php
@@ -423,7 +423,7 @@ class SendEmailAction implements Action {
     $this->newslettersRepository->flush();
   }
 
-  private function storeNewsletterOption(NewsletterEntity $newsletter, string $optionName, string $optionValue = null): void {
+  private function storeNewsletterOption(NewsletterEntity $newsletter, string $optionName, ?string $optionValue = null): void {
     $options = $newsletter->getOptions()->toArray();
     foreach ($options as $key => $option) {
       if ($option->getName() === $optionName) {
diff --git a/mailpoet/lib/Automation/Integrations/MailPoet/Analytics/Entities/Query.php b/mailpoet/lib/Automation/Integrations/MailPoet/Analytics/Entities/Query.php
index 7b6d04ee99..d1c4e75577 100644
--- a/mailpoet/lib/Automation/Integrations/MailPoet/Analytics/Entities/Query.php
+++ b/mailpoet/lib/Automation/Integrations/MailPoet/Analytics/Entities/Query.php
@@ -41,7 +41,7 @@ class Query {
     string $orderDirection = 'asc',
     int $page = 1,
     array $filter = [],
-    string $search = null
+    ?string $search = null
   ) {
     $this->primaryAfter = $primaryAfter;
     $this->primaryBefore = $primaryBefore;
diff --git a/mailpoet/lib/Automation/Integrations/MailPoet/Analytics/Entities/QueryWithCompare.php b/mailpoet/lib/Automation/Integrations/MailPoet/Analytics/Entities/QueryWithCompare.php
index bbf6fc1af4..f6dd1f0243 100644
--- a/mailpoet/lib/Automation/Integrations/MailPoet/Analytics/Entities/QueryWithCompare.php
+++ b/mailpoet/lib/Automation/Integrations/MailPoet/Analytics/Entities/QueryWithCompare.php
@@ -25,7 +25,7 @@ class QueryWithCompare extends Query {
     string $orderDirection = 'asc',
     int $page = 0,
     array $filter = [],
-    string $search = null
+    ?string $search = null
   ) {
     parent::__construct($primaryAfter, $primaryBefore, $limit, $orderBy, $orderDirection, $page, $filter, $search);
     $this->secondaryAfter = $secondaryAfter;
diff --git a/mailpoet/lib/Automation/Integrations/MailPoet/Fields/SubscriberAutomationFieldsFactory.php b/mailpoet/lib/Automation/Integrations/MailPoet/Fields/SubscriberAutomationFieldsFactory.php
index 22bebbc4fb..0a840b8322 100644
--- a/mailpoet/lib/Automation/Integrations/MailPoet/Fields/SubscriberAutomationFieldsFactory.php
+++ b/mailpoet/lib/Automation/Integrations/MailPoet/Fields/SubscriberAutomationFieldsFactory.php
@@ -66,7 +66,7 @@ class SubscriberAutomationFieldsFactory {
     ];
   }
 
-  private function getAutomationIds(SubscriberPayload $payload, array $status = null, array $params = []): array {
+  private function getAutomationIds(SubscriberPayload $payload, ?array $status = null, array $params = []): array {
     $inTheLastSeconds = isset($params['in_the_last']) ? (int)$params['in_the_last'] : null;
     $subject = new Subject(SubscriberSubject::KEY, ['subscriber_id' => $payload->getId()]);
     return $this->automationStorage->getAutomationIdsBySubject($subject, $status, $inTheLastSeconds);
diff --git a/mailpoet/lib/Automation/Integrations/MailPoet/Fields/SubscriberCustomFieldsFactory.php b/mailpoet/lib/Automation/Integrations/MailPoet/Fields/SubscriberCustomFieldsFactory.php
index fced812012..b207956a46 100644
--- a/mailpoet/lib/Automation/Integrations/MailPoet/Fields/SubscriberCustomFieldsFactory.php
+++ b/mailpoet/lib/Automation/Integrations/MailPoet/Fields/SubscriberCustomFieldsFactory.php
@@ -141,7 +141,7 @@ class SubscriberCustomFieldsFactory {
 
   private function getCustomFieldValue(SubscriberPayload $payload, CustomFieldEntity $customField): ?string {
     $subscriberCustomField = $payload->getSubscriber()->getSubscriberCustomFields()->filter(
-      function (SubscriberCustomFieldEntity $subscriberCustomField = null) use ($customField) {
+      function (?SubscriberCustomFieldEntity $subscriberCustomField = null) use ($customField) {
         return $subscriberCustomField && $subscriberCustomField->getCustomField() === $customField;
       }
     )->first() ?: null;
diff --git a/mailpoet/lib/Automation/Integrations/WooCommerce/Fields/CustomerOrderFieldsFactory.php b/mailpoet/lib/Automation/Integrations/WooCommerce/Fields/CustomerOrderFieldsFactory.php
index d54044fe4c..339120ef40 100644
--- a/mailpoet/lib/Automation/Integrations/WooCommerce/Fields/CustomerOrderFieldsFactory.php
+++ b/mailpoet/lib/Automation/Integrations/WooCommerce/Fields/CustomerOrderFieldsFactory.php
@@ -332,7 +332,7 @@ class CustomerOrderFieldsFactory {
     return $date ? new DateTimeImmutable($date, new DateTimeZone('GMT')) : null;
   }
 
-  private function getOrderProductTermIds(WC_Customer $customer, string $taxonomy, int $inTheLastSeconds = null): array {
+  private function getOrderProductTermIds(WC_Customer $customer, string $taxonomy, ?int $inTheLastSeconds = null): array {
     global $wpdb;
 
     $statuses = array_map(function (string $status) {
diff --git a/mailpoet/lib/Automation/Integrations/WooCommerce/Fields/CustomerReviewFieldsFactory.php b/mailpoet/lib/Automation/Integrations/WooCommerce/Fields/CustomerReviewFieldsFactory.php
index 4d56cdd595..2c66cadc61 100644
--- a/mailpoet/lib/Automation/Integrations/WooCommerce/Fields/CustomerReviewFieldsFactory.php
+++ b/mailpoet/lib/Automation/Integrations/WooCommerce/Fields/CustomerReviewFieldsFactory.php
@@ -53,7 +53,7 @@ class CustomerReviewFieldsFactory {
    * Calculate the customer's review count excluding multiple reviews on the same product.
    * Inspired by AutomateWoo implementation.
    */
-  private function getUniqueProductReviewCount(WC_Customer $customer, int $inTheLastSeconds = null): int {
+  private function getUniqueProductReviewCount(WC_Customer $customer, ?int $inTheLastSeconds = null): int {
     global $wpdb;
 
     $inTheLastFilter = isset($inTheLastSeconds) ? 'AND c.comment_date_gmt >= DATE_SUB(current_timestamp, INTERVAL %d SECOND)' : '';
diff --git a/mailpoet/lib/Automation/Integrations/WooCommerce/Payloads/CustomerPayload.php b/mailpoet/lib/Automation/Integrations/WooCommerce/Payloads/CustomerPayload.php
index c09b764832..e24509abeb 100644
--- a/mailpoet/lib/Automation/Integrations/WooCommerce/Payloads/CustomerPayload.php
+++ b/mailpoet/lib/Automation/Integrations/WooCommerce/Payloads/CustomerPayload.php
@@ -11,8 +11,8 @@ class CustomerPayload implements Payload {
   private ?WC_Order $order;
 
   public function __construct(
-    WC_Customer $customer = null,
-    WC_Order $order = null
+    ?WC_Customer $customer = null,
+    ?WC_Order $order = null
   ) {
     $this->customer = $customer;
     $this->order = $order;
diff --git a/mailpoet/lib/Captcha/CaptchaPhrase.php b/mailpoet/lib/Captcha/CaptchaPhrase.php
index 394c56bec9..2c895ff998 100644
--- a/mailpoet/lib/Captcha/CaptchaPhrase.php
+++ b/mailpoet/lib/Captcha/CaptchaPhrase.php
@@ -10,7 +10,7 @@ class CaptchaPhrase {
 
   public function __construct(
     CaptchaSession $session,
-    PhraseBuilder $phraseBuilder = null
+    ?PhraseBuilder $phraseBuilder = null
   ) {
     $this->session = $session;
     $this->phraseBuilder = $phraseBuilder ?? new PhraseBuilder();
diff --git a/mailpoet/lib/Captcha/Validator/ValidationError.php b/mailpoet/lib/Captcha/Validator/ValidationError.php
index 300ad10462..a6fe4a3ff7 100644
--- a/mailpoet/lib/Captcha/Validator/ValidationError.php
+++ b/mailpoet/lib/Captcha/Validator/ValidationError.php
@@ -10,7 +10,7 @@ class ValidationError extends \RuntimeException {
     $message = "",
     array $meta = [],
     $code = 0,
-    \Throwable $previous = null
+    ?\Throwable $previous = null
   ) {
     $this->meta = $meta;
     $this->meta['error'] = $message;
diff --git a/mailpoet/lib/Config/Capabilities.php b/mailpoet/lib/Config/Capabilities.php
index d55f3632f7..80e3c05f39 100644
--- a/mailpoet/lib/Config/Capabilities.php
+++ b/mailpoet/lib/Config/Capabilities.php
@@ -16,7 +16,7 @@ class Capabilities {
 
   public function __construct(
     $renderer = null,
-    WPFunctions $wp = null
+    ?WPFunctions $wp = null
   ) {
     if ($renderer !== null) {
       $this->renderer = $renderer;
diff --git a/mailpoet/lib/Config/HooksWooCommerce.php b/mailpoet/lib/Config/HooksWooCommerce.php
index 20049fd7f0..764a346185 100644
--- a/mailpoet/lib/Config/HooksWooCommerce.php
+++ b/mailpoet/lib/Config/HooksWooCommerce.php
@@ -145,7 +145,7 @@ class HooksWooCommerce {
     }
   }
 
-  public function onRegister($errors, string $userLogin, string $userEmail = null) {
+  public function onRegister($errors, string $userLogin, ?string $userEmail = null) {
     try {
       if (empty($errors->errors)) {
         $this->subscriberRegistration->onRegister($errors, $userLogin, $userEmail);
diff --git a/mailpoet/lib/Config/Menu.php b/mailpoet/lib/Config/Menu.php
index a9c2fd1c91..9358e4265d 100644
--- a/mailpoet/lib/Config/Menu.php
+++ b/mailpoet/lib/Config/Menu.php
@@ -711,7 +711,7 @@ class Menu {
     );
   }
 
-  public static function isOnMailPoetAdminPage(array $exclude = null, $screenId = null) {
+  public static function isOnMailPoetAdminPage(?array $exclude = null, $screenId = null) {
     if (is_null($screenId)) {
       if (empty($_REQUEST['page'])) {
         return false;
@@ -762,7 +762,7 @@ class Menu {
     // Used for displaying admin notices only
   }
 
-  public function checkPremiumKey(ServicesChecker $checker = null) {
+  public function checkPremiumKey(?ServicesChecker $checker = null) {
     $showNotices = isset($_SERVER['SCRIPT_NAME'])
       && stripos(sanitize_text_field(wp_unslash($_SERVER['SCRIPT_NAME'])), 'plugins.php') !== false;
     $checker = $checker ?: $this->servicesChecker;
diff --git a/mailpoet/lib/Config/TwigEnvironment.php b/mailpoet/lib/Config/TwigEnvironment.php
index 286ae2a656..1bea356a17 100644
--- a/mailpoet/lib/Config/TwigEnvironment.php
+++ b/mailpoet/lib/Config/TwigEnvironment.php
@@ -14,7 +14,7 @@ class TwigEnvironment extends Environment {
    * We need to produce the same class regardless of PHP_VERSION. Therefore, we
    * overwrite this method.
    **/
-  public function getTemplateClass(string $name, int $index = null): string {
+  public function getTemplateClass(string $name, ?int $index = null): string {
     return $this->templateClassPrefix . \hash('sha256', $name) . (null === $index ? '' : '___' . $index);
   }
 }
diff --git a/mailpoet/lib/Cron/DaemonHttpRunner.php b/mailpoet/lib/Cron/DaemonHttpRunner.php
index b8ac5a2c76..8e58033564 100644
--- a/mailpoet/lib/Cron/DaemonHttpRunner.php
+++ b/mailpoet/lib/Cron/DaemonHttpRunner.php
@@ -27,7 +27,7 @@ class DaemonHttpRunner {
   private $wordpressTrigger;
 
   public function __construct(
-    Daemon $daemon = null,
+    ?Daemon $daemon = null,
     CronHelper $cronHelper,
     SettingsController $settings,
     WordPress $wordpressTrigger
@@ -140,7 +140,7 @@ class DaemonHttpRunner {
    *
    * @return bool
    */
-  private function shouldTerminateExecution(array $settingsDaemonData = null) {
+  private function shouldTerminateExecution(?array $settingsDaemonData = null) {
     return !$settingsDaemonData ||
        $settingsDaemonData['token'] !== $this->token ||
        (isset($settingsDaemonData['status']) && $settingsDaemonData['status'] !== CronHelper::DAEMON_STATUS_ACTIVE);
diff --git a/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Links.php b/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Links.php
index dc16899c9e..0675d5b1d8 100644
--- a/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Links.php
+++ b/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Links.php
@@ -64,7 +64,7 @@ class Links {
     return $this->newsletterLinks->save($links, $newsletter->getId(), $queue->getId());
   }
 
-  public function getUnsubscribeUrl($queueId, SubscriberEntity $subscriber = null) {
+  public function getUnsubscribeUrl($queueId, ?SubscriberEntity $subscriber = null) {
     if ($this->trackingConfig->isEmailTrackingEnabled() && $subscriber) {
       $linkHash = $this->newsletterLinkRepository->findOneBy(
         [
diff --git a/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Mailer.php b/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Mailer.php
index 71d22b3e19..a0fa9aa106 100644
--- a/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Mailer.php
+++ b/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Mailer.php
@@ -23,7 +23,7 @@ class Mailer {
     $this->mailer = $this->configureMailer();
   }
 
-  public function configureMailer(NewsletterEntity $newsletter = null) {
+  public function configureMailer(?NewsletterEntity $newsletter = null) {
     $sender['address'] = ($newsletter && !empty($newsletter->getSenderAddress())) ?
       $newsletter->getSenderAddress() :
       null;
diff --git a/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Newsletter.php b/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Newsletter.php
index 9af577cc59..6f452a2b27 100644
--- a/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Newsletter.php
+++ b/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Newsletter.php
@@ -79,10 +79,10 @@ class Newsletter {
   private $personalizer;
 
   public function __construct(
-    WPFunctions $wp = null,
-    PostsTask $postsTask = null,
-    GATracking $gaTracking = null,
-    Emoji $emoji = null
+    ?WPFunctions $wp = null,
+    ?PostsTask $postsTask = null,
+    ?GATracking $gaTracking = null,
+    ?Emoji $emoji = null
   ) {
     $trackingConfig = ContainerWrapper::getInstance()->get(TrackingConfig::class);
     $this->trackingEnabled = $trackingConfig->isEmailTrackingEnabled();
diff --git a/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Shortcodes.php b/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Shortcodes.php
index a0e4b96ce2..c17775c0f9 100644
--- a/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Shortcodes.php
+++ b/mailpoet/lib/Cron/Workers/SendingQueue/Tasks/Shortcodes.php
@@ -16,7 +16,7 @@ class Shortcodes {
    * @param SubscriberEntity|null $subscriber
    * @param SendingQueueEntity|null $queue
    */
-  public static function process($content, $contentSource = null, NewsletterEntity $newsletter = null, SubscriberEntity $subscriber = null, SendingQueueEntity $queue = null) {
+  public static function process($content, $contentSource = null, ?NewsletterEntity $newsletter = null, ?SubscriberEntity $subscriber = null, ?SendingQueueEntity $queue = null) {
     /** @var NewsletterShortcodes $shortcodes */
     $shortcodes = ContainerWrapper::getInstance()->get(NewsletterShortcodes::class);
 
diff --git a/mailpoet/lib/Cron/Workers/StatsNotifications/Worker.php b/mailpoet/lib/Cron/Workers/StatsNotifications/Worker.php
index cf8064b377..dc8beae4f4 100644
--- a/mailpoet/lib/Cron/Workers/StatsNotifications/Worker.php
+++ b/mailpoet/lib/Cron/Workers/StatsNotifications/Worker.php
@@ -138,7 +138,7 @@ class Worker {
     ];
   }
 
-  private function prepareContext(NewsletterEntity $newsletter, SendingQueueEntity $sendingQueue, NewsletterLinkEntity $link = null) {
+  private function prepareContext(NewsletterEntity $newsletter, SendingQueueEntity $sendingQueue, ?NewsletterLinkEntity $link = null) {
     $statistics = $this->newsletterStatisticsRepository->getStatistics($newsletter);
     $clicked = ($statistics->getClickCount() * 100) / $statistics->getTotalSentCount();
     $opened = ($statistics->getOpenCount() * 100) / $statistics->getTotalSentCount();
diff --git a/mailpoet/lib/DI/ContainerConfigurator.php b/mailpoet/lib/DI/ContainerConfigurator.php
index 0fa11a7253..477e87cb53 100644
--- a/mailpoet/lib/DI/ContainerConfigurator.php
+++ b/mailpoet/lib/DI/ContainerConfigurator.php
@@ -699,7 +699,7 @@ class ContainerConfigurator implements IContainerConfigurator {
     return $container;
   }
 
-  public static function getPremiumService($id, ContainerInterface $container = null) {
+  public static function getPremiumService($id, ?ContainerInterface $container = null) {
     if ($container === null) {
       return null;
     }
diff --git a/mailpoet/lib/DI/ContainerWrapper.php b/mailpoet/lib/DI/ContainerWrapper.php
index 643cd50ffc..6550147bbd 100644
--- a/mailpoet/lib/DI/ContainerWrapper.php
+++ b/mailpoet/lib/DI/ContainerWrapper.php
@@ -20,7 +20,7 @@ class ContainerWrapper implements ContainerInterface {
 
   public function __construct(
     Container $freeContainer,
-    Container $premiumContainer = null
+    ?Container $premiumContainer = null
   ) {
     $this->freeContainer = $freeContainer;
     $this->premiumContainer = $premiumContainer;
diff --git a/mailpoet/lib/Doctrine/Repository.php b/mailpoet/lib/Doctrine/Repository.php
index 6fe09254f1..76a8b27463 100644
--- a/mailpoet/lib/Doctrine/Repository.php
+++ b/mailpoet/lib/Doctrine/Repository.php
@@ -41,7 +41,7 @@ abstract class Repository {
    * @param int|null $offset
    * @return T[]
    */
-  public function findBy(array $criteria, array $orderBy = null, $limit = null, $offset = null) {
+  public function findBy(array $criteria, ?array $orderBy = null, $limit = null, $offset = null) {
     return $this->doctrineRepository->findBy($criteria, $orderBy, $limit, $offset);
   }
 
@@ -62,7 +62,7 @@ abstract class Repository {
    * @param array|null $orderBy
    * @return T|null
    */
-  public function findOneBy(array $criteria, array $orderBy = null) {
+  public function findOneBy(array $criteria, ?array $orderBy = null) {
     return $this->doctrineRepository->findOneBy($criteria, $orderBy);
   }
 
@@ -114,7 +114,7 @@ abstract class Repository {
   /**
    * @param callable(T): bool|null $filter
    */
-  public function refreshAll(callable $filter = null): void {
+  public function refreshAll(?callable $filter = null): void {
     $entities = $this->getAllFromIdentityMap();
     foreach ($entities as $entity) {
       if ($filter && !$filter($entity)) {
@@ -142,7 +142,7 @@ abstract class Repository {
   /**
    * @param callable(T): bool|null $filter
    */
-  public function detachAll(callable $filter = null): void {
+  public function detachAll(?callable $filter = null): void {
     $entities = $this->getAllFromIdentityMap();
     foreach ($entities as $entity) {
       if ($filter && !$filter($entity)) {
diff --git a/mailpoet/lib/Entities/FormEntity.php b/mailpoet/lib/Entities/FormEntity.php
index 5e3e192fc4..bd8612ae23 100644
--- a/mailpoet/lib/Entities/FormEntity.php
+++ b/mailpoet/lib/Entities/FormEntity.php
@@ -174,7 +174,7 @@ class FormEntity {
     ];
   }
 
-  public function getBlocksByTypes(array $types, array $blocks = null): array {
+  public function getBlocksByTypes(array $types, ?array $blocks = null): array {
     $found = [];
     if ($blocks === null) {
       $blocks = $this->getBody() ?? [];
diff --git a/mailpoet/lib/Entities/NewsletterEntity.php b/mailpoet/lib/Entities/NewsletterEntity.php
index c90fd24495..470d136878 100644
--- a/mailpoet/lib/Entities/NewsletterEntity.php
+++ b/mailpoet/lib/Entities/NewsletterEntity.php
@@ -474,7 +474,7 @@ class NewsletterEntity {
    * @return int[]
    */
   public function getSegmentIds() {
-    return array_filter($this->newsletterSegments->map(function(NewsletterSegmentEntity $newsletterSegment = null) {
+    return array_filter($this->newsletterSegments->map(function(?NewsletterSegmentEntity $newsletterSegment = null) {
       if (!$newsletterSegment) return null;
       $segment = $newsletterSegment->getSegment();
       return $segment ? (int)$segment->getId() : null;
@@ -489,7 +489,7 @@ class NewsletterEntity {
   }
 
   public function getOption(string $name): ?NewsletterOptionEntity {
-    $option = $this->options->filter(function (NewsletterOptionEntity $option = null) use ($name): bool {
+    $option = $this->options->filter(function (?NewsletterOptionEntity $option = null) use ($name): bool {
       if (!$option) return false;
       return ($field = $option->getOptionField()) ? $field->getName() === $name : false;
     })->first();
diff --git a/mailpoet/lib/Entities/ScheduledTaskEntity.php b/mailpoet/lib/Entities/ScheduledTaskEntity.php
index 7acfe3cb9c..d44e23b326 100644
--- a/mailpoet/lib/Entities/ScheduledTaskEntity.php
+++ b/mailpoet/lib/Entities/ScheduledTaskEntity.php
@@ -243,7 +243,7 @@ class ScheduledTaskEntity {
   public function getSubscribersByProcessed(int $processed): array {
     $criteria = Criteria::create()
       ->where(Criteria::expr()->eq('processed', $processed));
-    $subscribers = $this->subscribers->matching($criteria)->map(function (ScheduledTaskSubscriberEntity $taskSubscriber = null): ?SubscriberEntity {
+    $subscribers = $this->subscribers->matching($criteria)->map(function (?ScheduledTaskSubscriberEntity $taskSubscriber = null): ?SubscriberEntity {
       if (!$taskSubscriber) return null;
       return $taskSubscriber->getSubscriber();
     });
diff --git a/mailpoet/lib/Entities/ScheduledTaskSubscriberEntity.php b/mailpoet/lib/Entities/ScheduledTaskSubscriberEntity.php
index 268b34a4e2..d9199619b1 100644
--- a/mailpoet/lib/Entities/ScheduledTaskSubscriberEntity.php
+++ b/mailpoet/lib/Entities/ScheduledTaskSubscriberEntity.php
@@ -61,7 +61,7 @@ class ScheduledTaskSubscriberEntity {
     SubscriberEntity $subscriber,
     int $processed = 0,
     int $failed = 0,
-    string $error = null
+    ?string $error = null
   ) {
     $this->task = $task;
     $this->subscriber = $subscriber;
diff --git a/mailpoet/lib/Entities/StatisticsNewsletterEntity.php b/mailpoet/lib/Entities/StatisticsNewsletterEntity.php
index 5dbd23a778..c8c80da81c 100644
--- a/mailpoet/lib/Entities/StatisticsNewsletterEntity.php
+++ b/mailpoet/lib/Entities/StatisticsNewsletterEntity.php
@@ -45,7 +45,7 @@ class StatisticsNewsletterEntity {
     NewsletterEntity $newsletter,
     SendingQueueEntity $queue,
     SubscriberEntity $subscriber,
-    \DateTimeInterface $sentAt = null
+    ?\DateTimeInterface $sentAt = null
   ) {
     $this->newsletter = $newsletter;
     $this->queue = $queue;
diff --git a/mailpoet/lib/Entities/StatisticsUnsubscribeEntity.php b/mailpoet/lib/Entities/StatisticsUnsubscribeEntity.php
index 99395ecd67..732e63b846 100644
--- a/mailpoet/lib/Entities/StatisticsUnsubscribeEntity.php
+++ b/mailpoet/lib/Entities/StatisticsUnsubscribeEntity.php
@@ -67,8 +67,8 @@ class StatisticsUnsubscribeEntity {
   private $method = self::METHOD_UNKNOWN;
 
   public function __construct(
-    NewsletterEntity $newsletter = null,
-    SendingQueueEntity $queue = null,
+    ?NewsletterEntity $newsletter = null,
+    ?SendingQueueEntity $queue = null,
     SubscriberEntity $subscriber
   ) {
     $this->newsletter = $newsletter;
diff --git a/mailpoet/lib/Entities/SubscriberEntity.php b/mailpoet/lib/Entities/SubscriberEntity.php
index 111c4f0db9..f3de869cef 100644
--- a/mailpoet/lib/Entities/SubscriberEntity.php
+++ b/mailpoet/lib/Entities/SubscriberEntity.php
@@ -504,7 +504,7 @@ class SubscriberEntity {
 
   /** * @return Collection<int, SegmentEntity> */
   public function getSegments() {
-    return $this->subscriberSegments->map(function (SubscriberSegmentEntity $subscriberSegment = null) {
+    return $this->subscriberSegments->map(function (?SubscriberSegmentEntity $subscriberSegment = null) {
       if (!$subscriberSegment) return null;
       return $subscriberSegment->getSegment();
     })->filter(function (?SegmentEntity $segment = null) {
@@ -635,7 +635,7 @@ class SubscriberEntity {
   /** @ORM\PreFlush */
   public function cleanupSubscriberSegments(): void {
     // Delete old orphan SubscriberSegments to avoid errors on update
-    $this->subscriberSegments->map(function (SubscriberSegmentEntity $subscriberSegment = null) {
+    $this->subscriberSegments->map(function (?SubscriberSegmentEntity $subscriberSegment = null) {
       if (!$subscriberSegment) return null;
       if ($subscriberSegment->getSegment() === null) {
         $this->subscriberSegments->removeElement($subscriberSegment);
diff --git a/mailpoet/lib/Form/Renderer.php b/mailpoet/lib/Form/Renderer.php
index 2cea3028ad..a36253f07e 100644
--- a/mailpoet/lib/Form/Renderer.php
+++ b/mailpoet/lib/Form/Renderer.php
@@ -41,14 +41,14 @@ class Renderer {
     return $html;
   }
 
-  public function renderHTML(FormEntity $form = null): string {
+  public function renderHTML(?FormEntity $form = null): string {
     if (($form instanceof FormEntity) && !empty($form->getBody()) && is_array($form->getSettings())) {
       return $this->renderBlocks($form->getBody(), $form->getSettings() ?? [], $form->getId());
     }
     return '';
   }
 
-  public function getCustomStyles(FormEntity $form = null): string {
+  public function getCustomStyles(?FormEntity $form = null): string {
     if (($form instanceof FormEntity) && (strlen(trim($form->getStyles() ?? '')) > 0)) {
       return strip_tags($form->getStyles() ?? '');
     } else {
diff --git a/mailpoet/lib/Listing/ListingDefinition.php b/mailpoet/lib/Listing/ListingDefinition.php
index 0cf79234e1..cad9212fd1 100644
--- a/mailpoet/lib/Listing/ListingDefinition.php
+++ b/mailpoet/lib/Listing/ListingDefinition.php
@@ -31,9 +31,9 @@ class ListingDefinition {
   private $selection;
 
   public function __construct(
-    string $group = null,
+    ?string $group = null,
     array $filters = [],
-    string $search = null,
+    ?string $search = null,
     array $parameters = [],
     string $sortBy = 'created_at',
     string $sortOrder = 'desc',
diff --git a/mailpoet/lib/Logging/LogRepository.php b/mailpoet/lib/Logging/LogRepository.php
index f904ea4362..1404b225b0 100644
--- a/mailpoet/lib/Logging/LogRepository.php
+++ b/mailpoet/lib/Logging/LogRepository.php
@@ -50,11 +50,11 @@ class LogRepository extends Repository {
    * @return LogEntity[]
    */
   public function getLogs(
-    \DateTimeInterface $dateFrom = null,
-    \DateTimeInterface $dateTo = null,
-    string $search = null,
-    string $offset = null,
-    string $limit = null
+    ?\DateTimeInterface $dateFrom = null,
+    ?\DateTimeInterface $dateTo = null,
+    ?string $search = null,
+    ?string $offset = null,
+    ?string $limit = null
   ): array {
     $query = $this->doctrineRepository->createQueryBuilder('l')
       ->select('l');
diff --git a/mailpoet/lib/Mailer/MailerFactory.php b/mailpoet/lib/Mailer/MailerFactory.php
index 87b77927a0..cabd1872b3 100644
--- a/mailpoet/lib/Mailer/MailerFactory.php
+++ b/mailpoet/lib/Mailer/MailerFactory.php
@@ -45,7 +45,7 @@ class MailerFactory {
     return $this->defaultMailer;
   }
 
-  public function buildMailer(array $mailerConfig = null, array $sender = null, array $replyTo = null, string $returnPath = null): Mailer {
+  public function buildMailer(?array $mailerConfig = null, ?array $sender = null, ?array $replyTo = null, ?string $returnPath = null): Mailer {
     $sender = $this->getSenderNameAndAddress($sender);
     $replyTo = $this->getReplyToNameAndAddress($sender, $replyTo);
     $mailerConfig = $mailerConfig ?? $this->getMailerConfig();
@@ -120,7 +120,7 @@ class MailerFactory {
     return $config;
   }
 
-  private function getSenderNameAndAddress(array $sender = null): array {
+  private function getSenderNameAndAddress(?array $sender = null): array {
     if (empty($sender)) {
       $sender = $this->settings->get('sender', []);
       if (empty($sender['address'])) throw new InvalidStateException(__('Sender name and email are not configured.', 'mailpoet'));
@@ -133,7 +133,7 @@ class MailerFactory {
     ];
   }
 
-  private function getReplyToNameAndAddress(array $sender, array $replyTo = null): array {
+  private function getReplyToNameAndAddress(array $sender, ?array $replyTo = null): array {
     if (!$replyTo) {
       $replyTo = $this->settings->get('reply_to');
       $replyTo['name'] = (!empty($replyTo['name'])) ?
diff --git a/mailpoet/lib/Mailer/MailerLog.php b/mailpoet/lib/Mailer/MailerLog.php
index 69d200c2ca..610e692afc 100644
--- a/mailpoet/lib/Mailer/MailerLog.php
+++ b/mailpoet/lib/Mailer/MailerLog.php
@@ -33,7 +33,7 @@ class MailerLog {
    * @param MailerLogData|null $mailerLog
    * @return MailerLogData
    */
-  public static function getMailerLog(array $mailerLog = null): array {
+  public static function getMailerLog(?array $mailerLog = null): array {
     if ($mailerLog) return $mailerLog;
     $settings = SettingsController::getInstance();
     $mailerLog = $settings->get(self::SETTING_NAME);
@@ -90,7 +90,7 @@ class MailerLog {
    * @return null
    * @throws \Exception
    */
-  public static function enforceExecutionRequirements(array $mailerLog = null) {
+  public static function enforceExecutionRequirements(?array $mailerLog = null) {
     $mailerLog = self::getMailerLog($mailerLog);
     if ($mailerLog['retry_attempt'] === self::RETRY_ATTEMPTS_LIMIT) {
       $mailerLog = self::pauseSending($mailerLog);
@@ -162,9 +162,9 @@ class MailerLog {
   public static function processError(
     string $operation,
     string $errorMessage,
-    string $errorCode = null,
+    ?string $errorCode = null,
     bool $pauseSending = false,
-    int $throttledBatchSize = null
+    ?int $throttledBatchSize = null
   ) {
     $mailerLog = self::getMailerLog();
     if (!isset($throttledBatchSize) || $throttledBatchSize === 1) {
@@ -232,7 +232,7 @@ class MailerLog {
     array $mailerLog,
     string $operation,
     string $errorMessage,
-    string $errorCode = null
+    ?string $errorCode = null
   ): array {
     $mailerLog['error'] = [
       'operation' => $operation,
@@ -248,7 +248,7 @@ class MailerLog {
    * @param MailerLogData|null $mailerLog
    * @return MailerLogError|null
    */
-  public static function getError(array $mailerLog = null): ?array {
+  public static function getError(?array $mailerLog = null): ?array {
     $mailerLog = self::getMailerLog($mailerLog);
     return isset($mailerLog['error']) ? $mailerLog['error'] : null;
   }
@@ -300,7 +300,7 @@ class MailerLog {
    * @param MailerLogData|null $mailerLog
    * @return bool
    */
-  public static function isSendingLimitReached(array $mailerLog = null): bool {
+  public static function isSendingLimitReached(?array $mailerLog = null): bool {
     $settings = SettingsController::getInstance();
     $mailerConfig = $settings->get(Mailer::MAILER_CONFIG_SETTING_NAME);
     // do not enforce sending limit for MailPoet's sending method
@@ -322,7 +322,7 @@ class MailerLog {
    * @param MailerLogData|null $mailerLog
    * @return int
    */
-  public static function sentSince(int $sinceSeconds = null, array $mailerLog = null): int {
+  public static function sentSince(?int $sinceSeconds = null, ?array $mailerLog = null): int {
 
     if ($sinceSeconds === null) {
       $settings = SettingsController::getInstance();
@@ -353,7 +353,7 @@ class MailerLog {
    * @param MailerLogData|null $mailerLog
    * @return MailerLogData
    */
-  private static function removeOutdatedSentInformationFromMailerlog(array $mailerLog = null): array {
+  private static function removeOutdatedSentInformationFromMailerlog(?array $mailerLog = null): array {
 
     $settings = SettingsController::getInstance();
     $mailerConfig = $settings->get(Mailer::MAILER_CONFIG_SETTING_NAME);
@@ -375,7 +375,7 @@ class MailerLog {
    * @param int|null $timestamp
    * @return string
    */
-  private static function sentEntriesDate(int $timestamp = null): string {
+  private static function sentEntriesDate(?int $timestamp = null): string {
 
     return date('Y-m-d H:i:s', $timestamp ?? time());
   }
@@ -384,7 +384,7 @@ class MailerLog {
    * @param MailerLogData|null $mailerLog
    * @return bool
    */
-  public static function isSendingPaused(array $mailerLog = null): bool {
+  public static function isSendingPaused(?array $mailerLog = null): bool {
     $mailerLog = self::getMailerLog($mailerLog);
     return $mailerLog['status'] === self::STATUS_PAUSED;
   }
@@ -393,7 +393,7 @@ class MailerLog {
    * @param MailerLogData|null $mailerLog
    * @return bool
    */
-  public static function isSendingWaitingForRetry(array $mailerLog = null): bool {
+  public static function isSendingWaitingForRetry(?array $mailerLog = null): bool {
     $mailerLog = self::getMailerLog($mailerLog);
     $retryAt = $mailerLog['retry_at'] ?? null;
     return $retryAt && (time() <= $retryAt);
diff --git a/mailpoet/lib/Mailer/MetaInfo.php b/mailpoet/lib/Mailer/MetaInfo.php
index beea55ad24..52930fc3f5 100644
--- a/mailpoet/lib/Mailer/MetaInfo.php
+++ b/mailpoet/lib/Mailer/MetaInfo.php
@@ -18,7 +18,7 @@ class MetaInfo {
     return $this->makeMetaInfo('email_stats_notification', 'unknown', 'administrator');
   }
 
-  public function getWordPressTransactionalMetaInfo(SubscriberEntity $subscriber = null) {
+  public function getWordPressTransactionalMetaInfo(?SubscriberEntity $subscriber = null) {
     return $this->makeMetaInfo(
       'transactional',
       $subscriber ? $subscriber->getStatus() : 'unknown',
diff --git a/mailpoet/lib/Mailer/Methods/Common/BlacklistCheck.php b/mailpoet/lib/Mailer/Methods/Common/BlacklistCheck.php
index 3b004c5430..043809b271 100644
--- a/mailpoet/lib/Mailer/Methods/Common/BlacklistCheck.php
+++ b/mailpoet/lib/Mailer/Methods/Common/BlacklistCheck.php
@@ -9,7 +9,7 @@ class BlacklistCheck {
   private $blacklist;
 
   public function __construct(
-    Blacklist $blacklist = null
+    ?Blacklist $blacklist = null
   ) {
     if (is_null($blacklist)) {
       $blacklist = new Blacklist();
diff --git a/mailpoet/lib/Migrator/Migrator.php b/mailpoet/lib/Migrator/Migrator.php
index a721f6a270..a45310a43c 100644
--- a/mailpoet/lib/Migrator/Migrator.php
+++ b/mailpoet/lib/Migrator/Migrator.php
@@ -30,7 +30,7 @@ class Migrator {
     $this->store = $store;
   }
 
-  public function run(Logger $logger = null): void {
+  public function run(?Logger $logger = null): void {
     $this->store->ensureMigrationsTable();
     $migrations = $this->getStatus();
 
diff --git a/mailpoet/lib/Newsletter/Editor/PostContentManager.php b/mailpoet/lib/Newsletter/Editor/PostContentManager.php
index 84a62e2895..87ebd215dc 100644
--- a/mailpoet/lib/Newsletter/Editor/PostContentManager.php
+++ b/mailpoet/lib/Newsletter/Editor/PostContentManager.php
@@ -18,7 +18,7 @@ class PostContentManager {
   private $wp;
 
   public function __construct(
-    WooCommerceHelper $woocommerceHelper = null
+    ?WooCommerceHelper $woocommerceHelper = null
   ) {
     $this->wp = new WPFunctions;
     $this->maxExcerptLength = $this->wp->applyFilters('mailpoet_newsletter_post_excerpt_length', $this->maxExcerptLength);
diff --git a/mailpoet/lib/Newsletter/Editor/PostTransformer.php b/mailpoet/lib/Newsletter/Editor/PostTransformer.php
index 2434bd1ce9..1fa236d678 100644
--- a/mailpoet/lib/Newsletter/Editor/PostTransformer.php
+++ b/mailpoet/lib/Newsletter/Editor/PostTransformer.php
@@ -15,7 +15,7 @@ class PostTransformer {
 
   public function __construct(
     $args,
-    PostTransformerContentsExtractor $extractor = null
+    ?PostTransformerContentsExtractor $extractor = null
   ) {
     $this->args = $args;
     $this->withLayout = isset($args['withLayout']) ? (bool)filter_var($args['withLayout'], FILTER_VALIDATE_BOOLEAN) : false;
diff --git a/mailpoet/lib/Newsletter/Listing/NewsletterListingRepository.php b/mailpoet/lib/Newsletter/Listing/NewsletterListingRepository.php
index 365c05cf62..89200bb5db 100644
--- a/mailpoet/lib/Newsletter/Listing/NewsletterListingRepository.php
+++ b/mailpoet/lib/Newsletter/Listing/NewsletterListingRepository.php
@@ -260,7 +260,7 @@ class NewsletterListingRepository extends ListingRepository {
     $queryBuilder->addOrderBy("n.$sortBy", $sortOrder);
   }
 
-  private function applyType(QueryBuilder $queryBuilder, string $type, string $group = null) {
+  private function applyType(QueryBuilder $queryBuilder, string $type, ?string $group = null) {
     if (!in_array($type, self::$supportedTypes)) {
       return;
     }
diff --git a/mailpoet/lib/Newsletter/Renderer/Blocks/AbandonedCartContent.php b/mailpoet/lib/Newsletter/Renderer/Blocks/AbandonedCartContent.php
index 39de335307..3471983855 100644
--- a/mailpoet/lib/Newsletter/Renderer/Blocks/AbandonedCartContent.php
+++ b/mailpoet/lib/Newsletter/Renderer/Blocks/AbandonedCartContent.php
@@ -22,7 +22,7 @@ class AbandonedCartContent {
     NewsletterEntity $newsletter,
     array $args,
     bool $preview = false,
-    SendingQueueEntity $sendingQueue = null
+    ?SendingQueueEntity $sendingQueue = null
   ): array {
     if (
       !in_array(
@@ -38,12 +38,12 @@ class AbandonedCartContent {
       // Do not display the block if not an automatic email
       return [];
     }
-    $groupOption = $newsletter->getOptions()->filter(function (NewsletterOptionEntity $newsletterOption = null) {
+    $groupOption = $newsletter->getOptions()->filter(function (?NewsletterOptionEntity $newsletterOption = null) {
       if (!$newsletterOption) return false;
       $optionField = $newsletterOption->getOptionField();
       return $optionField && $optionField->getName() === 'group';
     })->first();
-    $eventOption = $newsletter->getOptions()->filter(function (NewsletterOptionEntity $newsletterOption = null) {
+    $eventOption = $newsletter->getOptions()->filter(function (?NewsletterOptionEntity $newsletterOption = null) {
       if (!$newsletterOption) return false;
       $optionField = $newsletterOption->getOptionField();
       return $optionField && $optionField->getName() === 'event';
diff --git a/mailpoet/lib/Newsletter/Renderer/Preprocessor.php b/mailpoet/lib/Newsletter/Renderer/Preprocessor.php
index e23b68d22f..c5b6c8242b 100644
--- a/mailpoet/lib/Newsletter/Renderer/Preprocessor.php
+++ b/mailpoet/lib/Newsletter/Renderer/Preprocessor.php
@@ -48,7 +48,7 @@ class Preprocessor {
    * @param NewsletterEntity $newsletter
    * @return array
    */
-  public function process(NewsletterEntity $newsletter, $content, bool $preview = false, SendingQueueEntity $sendingQueue = null) {
+  public function process(NewsletterEntity $newsletter, $content, bool $preview = false, ?SendingQueueEntity $sendingQueue = null) {
     if (!array_key_exists('blocks', $content)) {
       return $content;
     }
@@ -74,7 +74,7 @@ class Preprocessor {
     return $containerBlocks;
   }
 
-  public function processBlock(NewsletterEntity $newsletter, array $block, bool $preview = false, SendingQueueEntity $sendingQueue = null): array {
+  public function processBlock(NewsletterEntity $newsletter, array $block, bool $preview = false, ?SendingQueueEntity $sendingQueue = null): array {
     switch ($block['type']) {
       case 'abandonedCartContent':
         return $this->abandonedCartContent->render($newsletter, $block, $preview, $sendingQueue);
diff --git a/mailpoet/lib/Newsletter/Renderer/Renderer.php b/mailpoet/lib/Newsletter/Renderer/Renderer.php
index b1ed530969..d7b051867f 100644
--- a/mailpoet/lib/Newsletter/Renderer/Renderer.php
+++ b/mailpoet/lib/Newsletter/Renderer/Renderer.php
@@ -68,7 +68,7 @@ class Renderer {
     $this->capabilitiesManager = $capabilitiesManager;
   }
 
-  public function render(NewsletterEntity $newsletter, SendingQueueEntity $sendingQueue = null, $type = false) {
+  public function render(NewsletterEntity $newsletter, ?SendingQueueEntity $sendingQueue = null, $type = false) {
     return $this->_render($newsletter, $sendingQueue, $type);
   }
 
@@ -76,7 +76,7 @@ class Renderer {
     return $this->_render($newsletter, null, $type, true, $subject);
   }
 
-  private function _render(NewsletterEntity $newsletter, SendingQueueEntity $sendingQueue = null, $type = false, $preview = false, $subject = null) {
+  private function _render(NewsletterEntity $newsletter, ?SendingQueueEntity $sendingQueue = null, $type = false, $preview = false, $subject = null) {
     $language = $this->wp->getBloginfo('language');
     $metaRobots = $preview ? '<meta name="robots" content="noindex, nofollow" />' : '';
     $subject = $subject ?: $newsletter->getSubject();
diff --git a/mailpoet/lib/Newsletter/Shortcodes/Categories/CategoryInterface.php b/mailpoet/lib/Newsletter/Shortcodes/Categories/CategoryInterface.php
index 620b0e4729..d1d4f5633b 100644
--- a/mailpoet/lib/Newsletter/Shortcodes/Categories/CategoryInterface.php
+++ b/mailpoet/lib/Newsletter/Shortcodes/Categories/CategoryInterface.php
@@ -9,9 +9,9 @@ use MailPoet\Entities\SubscriberEntity;
 interface CategoryInterface {
   public function process(
     array $shortcodeDetails,
-    NewsletterEntity $newsletter = null,
-    SubscriberEntity $subscriber = null,
-    SendingQueueEntity $queue = null,
+    ?NewsletterEntity $newsletter = null,
+    ?SubscriberEntity $subscriber = null,
+    ?SendingQueueEntity $queue = null,
     string $content = '',
     bool $wpUserPreview = false
   ): ?string;
diff --git a/mailpoet/lib/Newsletter/Shortcodes/Categories/Date.php b/mailpoet/lib/Newsletter/Shortcodes/Categories/Date.php
index 674e965186..e53eefe4a1 100644
--- a/mailpoet/lib/Newsletter/Shortcodes/Categories/Date.php
+++ b/mailpoet/lib/Newsletter/Shortcodes/Categories/Date.php
@@ -10,9 +10,9 @@ use MailPoet\WP\Functions as WPFunctions;
 class Date implements CategoryInterface {
   public function process(
     array $shortcodeDetails,
-    NewsletterEntity $newsletter = null,
-    SubscriberEntity $subscriber = null,
-    SendingQueueEntity $queue = null,
+    ?NewsletterEntity $newsletter = null,
+    ?SubscriberEntity $subscriber = null,
+    ?SendingQueueEntity $queue = null,
     string $content = '',
     bool $wpUserPreview = false
   ): ?string {
diff --git a/mailpoet/lib/Newsletter/Shortcodes/Categories/Link.php b/mailpoet/lib/Newsletter/Shortcodes/Categories/Link.php
index 150c9c9d43..18b05a84a7 100644
--- a/mailpoet/lib/Newsletter/Shortcodes/Categories/Link.php
+++ b/mailpoet/lib/Newsletter/Shortcodes/Categories/Link.php
@@ -40,9 +40,9 @@ class Link implements CategoryInterface {
 
   public function process(
     array $shortcodeDetails,
-    NewsletterEntity $newsletter = null,
-    SubscriberEntity $subscriber = null,
-    SendingQueueEntity $queue = null,
+    ?NewsletterEntity $newsletter = null,
+    ?SubscriberEntity $subscriber = null,
+    ?SendingQueueEntity $queue = null,
     string $content = '',
     bool $wpUserPreview = false
   ): ?string {
@@ -119,9 +119,9 @@ class Link implements CategoryInterface {
 
   public function processShortcodeAction(
     $shortcodeAction,
-    NewsletterEntity $newsletter = null,
-    SubscriberEntity $subscriber = null,
-    SendingQueueEntity $queue = null,
+    ?NewsletterEntity $newsletter = null,
+    ?SubscriberEntity $subscriber = null,
+    ?SendingQueueEntity $queue = null,
     $wpUserPreview = false
   ): ?string {
     $subscriptionUrlFactory = SubscriptionUrlFactory::getInstance();
diff --git a/mailpoet/lib/Newsletter/Shortcodes/Categories/Newsletter.php b/mailpoet/lib/Newsletter/Shortcodes/Categories/Newsletter.php
index fe4b07ede3..957c0351f3 100644
--- a/mailpoet/lib/Newsletter/Shortcodes/Categories/Newsletter.php
+++ b/mailpoet/lib/Newsletter/Shortcodes/Categories/Newsletter.php
@@ -25,9 +25,9 @@ class Newsletter implements CategoryInterface {
 
   public function process(
     array $shortcodeDetails,
-    NewsletterEntity $newsletter = null,
-    SubscriberEntity $subscriber = null,
-    SendingQueueEntity $queue = null,
+    ?NewsletterEntity $newsletter = null,
+    ?SubscriberEntity $subscriber = null,
+    ?SendingQueueEntity $queue = null,
     string $content = '',
     bool $wpUserPreview = false
   ): ?string {
diff --git a/mailpoet/lib/Newsletter/Shortcodes/Categories/Site.php b/mailpoet/lib/Newsletter/Shortcodes/Categories/Site.php
index 9996518b43..39042d0f88 100644
--- a/mailpoet/lib/Newsletter/Shortcodes/Categories/Site.php
+++ b/mailpoet/lib/Newsletter/Shortcodes/Categories/Site.php
@@ -19,9 +19,9 @@ class Site implements CategoryInterface {
 
   public function process(
     array $shortcodeDetails,
-    NewsletterEntity $newsletter = null,
-    SubscriberEntity $subscriber = null,
-    SendingQueueEntity $queue = null,
+    ?NewsletterEntity $newsletter = null,
+    ?SubscriberEntity $subscriber = null,
+    ?SendingQueueEntity $queue = null,
     string $content = '',
     bool $wpUserPreview = false
   ): ?string {
diff --git a/mailpoet/lib/Newsletter/Shortcodes/Categories/Subscriber.php b/mailpoet/lib/Newsletter/Shortcodes/Categories/Subscriber.php
index e87e245c78..17dfa30e06 100644
--- a/mailpoet/lib/Newsletter/Shortcodes/Categories/Subscriber.php
+++ b/mailpoet/lib/Newsletter/Shortcodes/Categories/Subscriber.php
@@ -28,9 +28,9 @@ class Subscriber implements CategoryInterface {
 
   public function process(
     array $shortcodeDetails,
-    NewsletterEntity $newsletter = null,
-    SubscriberEntity $subscriber = null,
-    SendingQueueEntity $queue = null,
+    ?NewsletterEntity $newsletter = null,
+    ?SubscriberEntity $subscriber = null,
+    ?SendingQueueEntity $queue = null,
     string $content = '',
     bool $wpUserPreview = false
   ): ?string {
diff --git a/mailpoet/lib/Newsletter/Shortcodes/Shortcodes.php b/mailpoet/lib/Newsletter/Shortcodes/Shortcodes.php
index c085cd0c63..bc48d270e1 100644
--- a/mailpoet/lib/Newsletter/Shortcodes/Shortcodes.php
+++ b/mailpoet/lib/Newsletter/Shortcodes/Shortcodes.php
@@ -60,15 +60,15 @@ class Shortcodes {
     $this->wp = $wp;
   }
 
-  public function setNewsletter(NewsletterEntity $newsletter = null): void {
+  public function setNewsletter(?NewsletterEntity $newsletter = null): void {
     $this->newsletter = $newsletter;
   }
 
-  public function setSubscriber(SubscriberEntity $subscriber = null): void {
+  public function setSubscriber(?SubscriberEntity $subscriber = null): void {
     $this->subscriber = $subscriber;
   }
 
-  public function setQueue(SendingQueueEntity $queue = null): void {
+  public function setQueue(?SendingQueueEntity $queue = null): void {
     $this->queue = $queue;
   }
 
diff --git a/mailpoet/lib/Newsletter/Statistics/NewsletterStatisticsRepository.php b/mailpoet/lib/Newsletter/Statistics/NewsletterStatisticsRepository.php
index 65a43d0575..8bf73c9eb9 100644
--- a/mailpoet/lib/Newsletter/Statistics/NewsletterStatisticsRepository.php
+++ b/mailpoet/lib/Newsletter/Statistics/NewsletterStatisticsRepository.php
@@ -65,8 +65,8 @@ class NewsletterStatisticsRepository extends Repository {
    */
   public function getBatchStatistics(
     array $newsletters,
-    \DateTimeImmutable $from = null,
-    \DateTimeImmutable $to = null,
+    ?\DateTimeImmutable $from = null,
+    ?\DateTimeImmutable $to = null,
     array $include = [
       'totals',
       StatisticsClickEntity::class,
@@ -133,8 +133,8 @@ class NewsletterStatisticsRepository extends Repository {
    */
   public function getAllForSubscriber(
     SubscriberEntity $subscriber,
-    int $limit = null,
-    int $offset = null
+    ?int $limit = null,
+    ?int $offset = null
   ): array {
     return $this->entityManager->createQueryBuilder()
       ->select('IDENTITY(statistics.newsletter) AS newsletter_id')
@@ -192,7 +192,7 @@ class NewsletterStatisticsRepository extends Repository {
     }
   }
 
-  private function getTotalSentCounts(array $newsletters, \DateTimeImmutable $from = null, \DateTimeImmutable $to = null): array {
+  private function getTotalSentCounts(array $newsletters, ?\DateTimeImmutable $from = null, ?\DateTimeImmutable $to = null): array {
     $query = $this->doctrineRepository
       ->createQueryBuilder('n')
       ->select('n.id, SUM(q.countProcessed) AS cnt')
@@ -226,7 +226,7 @@ class NewsletterStatisticsRepository extends Repository {
     return $counts;
   }
 
-  private function getStatisticCounts(string $statisticsEntityName, array $newsletters, \DateTimeImmutable $from = null, \DateTimeImmutable $to = null): array {
+  private function getStatisticCounts(string $statisticsEntityName, array $newsletters, ?\DateTimeImmutable $from = null, ?\DateTimeImmutable $to = null): array {
     $qb = $this->getStatisticsQuery($statisticsEntityName, $newsletters);
     if (
       $statisticsEntityName === StatisticsClickEntity::class
@@ -267,7 +267,7 @@ class NewsletterStatisticsRepository extends Repository {
       ->setParameter('newsletters', $newsletters);
   }
 
-  private function getWooCommerceRevenues(array $newsletters, \DateTimeImmutable $from = null, \DateTimeImmutable $to = null) {
+  private function getWooCommerceRevenues(array $newsletters, ?\DateTimeImmutable $from = null, ?\DateTimeImmutable $to = null) {
     if (!$this->wcHelper->isWooCommerceActive()) {
       return null;
     }
diff --git a/mailpoet/lib/Newsletter/ViewInBrowser/ViewInBrowserRenderer.php b/mailpoet/lib/Newsletter/ViewInBrowser/ViewInBrowserRenderer.php
index 5218c82661..ac2f9ca110 100644
--- a/mailpoet/lib/Newsletter/ViewInBrowser/ViewInBrowserRenderer.php
+++ b/mailpoet/lib/Newsletter/ViewInBrowser/ViewInBrowserRenderer.php
@@ -49,8 +49,8 @@ class ViewInBrowserRenderer {
   public function render(
     bool $isPreview,
     NewsletterEntity $newsletter,
-    SubscriberEntity $subscriber = null,
-    SendingQueueEntity $queue = null
+    ?SubscriberEntity $subscriber = null,
+    ?SendingQueueEntity $queue = null
   ) {
     $wpUserPreview = $isPreview;
     $isTrackingEnabled = $this->trackingConfig->isEmailTrackingEnabled();
diff --git a/mailpoet/lib/Segments/DynamicSegments/Filters/EmailAction.php b/mailpoet/lib/Segments/DynamicSegments/Filters/EmailAction.php
index 96798c7d18..1bb63cba3c 100644
--- a/mailpoet/lib/Segments/DynamicSegments/Filters/EmailAction.php
+++ b/mailpoet/lib/Segments/DynamicSegments/Filters/EmailAction.php
@@ -191,7 +191,7 @@ class EmailAction implements Filter {
     return $queryBuilder;
   }
 
-  private function createNotStatsJoinCondition(string $parameterSuffix, array $linkIds = null): string {
+  private function createNotStatsJoinCondition(string $parameterSuffix, ?array $linkIds = null): string {
     $clause = "statssent.subscriber_id = stats.subscriber_id AND stats.newsletter_id = :newsletter" . $parameterSuffix;
     if ($linkIds) {
       $clause .= ' AND stats.link_id IN (:links' . $parameterSuffix . ')';
diff --git a/mailpoet/lib/Segments/DynamicSegments/Filters/WooFilterHelper.php b/mailpoet/lib/Segments/DynamicSegments/Filters/WooFilterHelper.php
index 41e033c261..a32e438ae6 100644
--- a/mailpoet/lib/Segments/DynamicSegments/Filters/WooFilterHelper.php
+++ b/mailpoet/lib/Segments/DynamicSegments/Filters/WooFilterHelper.php
@@ -73,7 +73,7 @@ class WooFilterHelper {
    * @param array|null $allowedStatuses
    * @return string - The alias of the joined order stats table
    */
-  public function applyOrderStatusFilter(QueryBuilder $queryBuilder, array $allowedStatuses = null): string {
+  public function applyOrderStatusFilter(QueryBuilder $queryBuilder, ?array $allowedStatuses = null): string {
     if (is_null($allowedStatuses)) {
       $allowedStatuses = $this->defaultIncludedStatuses();
     }
diff --git a/mailpoet/lib/Segments/SegmentSubscribersRepository.php b/mailpoet/lib/Segments/SegmentSubscribersRepository.php
index f1fec70436..75212b85cb 100644
--- a/mailpoet/lib/Segments/SegmentSubscribersRepository.php
+++ b/mailpoet/lib/Segments/SegmentSubscribersRepository.php
@@ -38,7 +38,7 @@ class SegmentSubscribersRepository {
     $this->segmentsRepository = $segmentsRepository;
   }
 
-  public function findSubscribersIdsInSegment(int $segmentId, array $candidateIds = null): array {
+  public function findSubscribersIdsInSegment(int $segmentId, ?array $candidateIds = null): array {
     return $this->loadSubscriberIdsInSegment($segmentId, $candidateIds);
   }
 
@@ -46,13 +46,13 @@ class SegmentSubscribersRepository {
     return $this->loadSubscriberIdsInSegment($segmentId);
   }
 
-  public function getSubscribersCount(int $segmentId, string $status = null): int {
+  public function getSubscribersCount(int $segmentId, ?string $status = null): int {
     $segment = $this->getSegment($segmentId);
     $result = $this->getSubscribersStatisticsCount($segment);
     return (int)$result[$status ?: 'all'];
   }
 
-  public function getSubscribersCountBySegmentIds(array $segmentIds, string $status = null, ?int $filterSegmentId = null): int {
+  public function getSubscribersCountBySegmentIds(array $segmentIds, ?string $status = null, ?int $filterSegmentId = null): int {
     $segmentRepository = $this->entityManager->getRepository(SegmentEntity::class);
     $segments = $segmentRepository->findBy(['id' => $segmentIds]);
     $subscribersTable = $this->entityManager->getClassMetadata(SubscriberEntity::class)->getTableName();
@@ -369,7 +369,7 @@ class SegmentSubscribersRepository {
       ->setParameter('statusSubscribed', SubscriberEntity::STATUS_SUBSCRIBED);
   }
 
-  private function loadSubscriberIdsInSegment(int $segmentId, array $candidateIds = null): array {
+  private function loadSubscriberIdsInSegment(int $segmentId, ?array $candidateIds = null): array {
     $segment = $this->getSegment($segmentId);
     $subscribersTable = $this->entityManager->getClassMetadata(SubscriberEntity::class)->getTableName();
     $queryBuilder = $this->entityManager
@@ -397,7 +397,7 @@ class SegmentSubscribersRepository {
   private function filterSubscribersInStaticSegment(
     QueryBuilder $queryBuilder,
     SegmentEntity $segment,
-    string $status = null
+    ?string $status = null
   ): QueryBuilder {
     $subscribersSegmentsTable = $this->entityManager->getClassMetadata(SubscriberSegmentEntity::class)->getTableName();
     $subscribersTable = $this->entityManager->getClassMetadata(SubscriberEntity::class)->getTableName();
@@ -420,7 +420,7 @@ class SegmentSubscribersRepository {
   private function filterSubscribersInDynamicSegment(
     QueryBuilder $queryBuilder,
     SegmentEntity $segment,
-    string $status = null
+    ?string $status = null
   ): QueryBuilder {
     $filters = [];
     $dynamicFilters = $segment->getDynamicFilters();
diff --git a/mailpoet/lib/Segments/SegmentsSimpleListRepository.php b/mailpoet/lib/Segments/SegmentsSimpleListRepository.php
index f5511158e9..a3f794372f 100644
--- a/mailpoet/lib/Segments/SegmentsSimpleListRepository.php
+++ b/mailpoet/lib/Segments/SegmentsSimpleListRepository.php
@@ -65,7 +65,7 @@ class SegmentsSimpleListRepository {
    */
   private function getList(
     array $segmentTypes = [],
-    string $subscriberGlobalStatus = null
+    ?string $subscriberGlobalStatus = null
   ): array {
     $segmentsTable = $this->entityManager->getClassMetadata(SegmentEntity::class)->getTableName();
 
diff --git a/mailpoet/lib/Services/AuthorizedEmailsController.php b/mailpoet/lib/Services/AuthorizedEmailsController.php
index 71e10891cc..95ba2b475b 100644
--- a/mailpoet/lib/Services/AuthorizedEmailsController.php
+++ b/mailpoet/lib/Services/AuthorizedEmailsController.php
@@ -161,7 +161,7 @@ class AuthorizedEmailsController {
     return null;
   }
 
-  public function onNewsletterSenderAddressUpdate(NewsletterEntity $newsletter, string $oldSenderAddress = null) {
+  public function onNewsletterSenderAddressUpdate(NewsletterEntity $newsletter, ?string $oldSenderAddress = null) {
     if ($newsletter->getSenderAddress() === $oldSenderAddress) {
       return;
     }
@@ -243,7 +243,7 @@ class AuthorizedEmailsController {
   /**
    * @param array|null $error
    */
-  private function updateMailerLog(array $error = null) {
+  private function updateMailerLog(?array $error = null) {
     if ($error) {
       return;
     }
diff --git a/mailpoet/lib/Services/Bridge.php b/mailpoet/lib/Services/Bridge.php
index de3571ea76..632e96fadb 100644
--- a/mailpoet/lib/Services/Bridge.php
+++ b/mailpoet/lib/Services/Bridge.php
@@ -52,7 +52,7 @@ class Bridge {
   private $settings;
 
   public function __construct(
-    SettingsController $settingsController = null
+    ?SettingsController $settingsController = null
   ) {
     if ($settingsController === null) {
       $settingsController = SettingsController::getInstance();
diff --git a/mailpoet/lib/Settings/TrackingConfig.php b/mailpoet/lib/Settings/TrackingConfig.php
index 81e80bf0f3..d5d9dd2526 100644
--- a/mailpoet/lib/Settings/TrackingConfig.php
+++ b/mailpoet/lib/Settings/TrackingConfig.php
@@ -19,22 +19,22 @@ class TrackingConfig {
     $this->settings = $settings;
   }
 
-  public function isEmailTrackingEnabled(string $level = null): bool {
+  public function isEmailTrackingEnabled(?string $level = null): bool {
     $level = $level ?? $this->settings->get('tracking.level', self::LEVEL_FULL);
     return in_array($level, [self::LEVEL_PARTIAL, self::LEVEL_FULL], true);
   }
 
-  public function isCookieTrackingEnabled(string $level = null): bool {
+  public function isCookieTrackingEnabled(?string $level = null): bool {
     $level = $level ?? $this->settings->get('tracking.level', self::LEVEL_FULL);
     return $level === self::LEVEL_FULL;
   }
 
-  public function areOpensMerged(string $opens = null): bool {
+  public function areOpensMerged(?string $opens = null): bool {
     $opens = $opens ?? $this->settings->get('tracking.opens', self::OPENS_MERGED);
     return $opens !== self::OPENS_SEPARATED;
   }
 
-  public function areOpensSeparated(string $opens = null): bool {
+  public function areOpensSeparated(?string $opens = null): bool {
     return !$this->areOpensMerged($opens);
   }
 
diff --git a/mailpoet/lib/Statistics/Track/Unsubscribes.php b/mailpoet/lib/Statistics/Track/Unsubscribes.php
index a71b94ecc2..dff9a85ca3 100644
--- a/mailpoet/lib/Statistics/Track/Unsubscribes.php
+++ b/mailpoet/lib/Statistics/Track/Unsubscribes.php
@@ -35,8 +35,8 @@ class Unsubscribes {
   public function track(
     int $subscriberId,
     string $source,
-    int $queueId = null,
-    string $meta = null,
+    ?int $queueId = null,
+    ?string $meta = null,
     string $method = StatisticsUnsubscribeEntity::METHOD_UNKNOWN
   ) {
     $queue = null;
diff --git a/mailpoet/lib/Subscribers/RequiredCustomFieldValidator.php b/mailpoet/lib/Subscribers/RequiredCustomFieldValidator.php
index 4988024056..4893ff2d0a 100644
--- a/mailpoet/lib/Subscribers/RequiredCustomFieldValidator.php
+++ b/mailpoet/lib/Subscribers/RequiredCustomFieldValidator.php
@@ -22,7 +22,7 @@ class RequiredCustomFieldValidator {
    *
    * @throws Exception
    */
-  public function validate(array $data, FormEntity $form = null) {
+  public function validate(array $data, ?FormEntity $form = null) {
     $allCustomFields = $this->getCustomFields($form);
     foreach ($allCustomFields as $customFieldId => $customFieldName) {
       if ($this->isCustomFieldMissing($customFieldId, $data)) {
@@ -47,7 +47,7 @@ class RequiredCustomFieldValidator {
     return false;
   }
 
-  private function getCustomFields(FormEntity $form = null): array {
+  private function getCustomFields(?FormEntity $form = null): array {
     $result = [];
 
     if ($form) {
diff --git a/mailpoet/lib/Subscription/Blacklist.php b/mailpoet/lib/Subscription/Blacklist.php
index 99395e3709..2f36f266f8 100644
--- a/mailpoet/lib/Subscription/Blacklist.php
+++ b/mailpoet/lib/Subscription/Blacklist.php
@@ -20,8 +20,8 @@ class Blacklist {
   ];
 
   public function __construct(
-    array $blacklistedEmails = null,
-    array $blacklistedDomains = null
+    ?array $blacklistedEmails = null,
+    ?array $blacklistedDomains = null
   ) {
     if ($blacklistedEmails) {
       $this->blacklistedEmails = array_fill_keys(array_map([$this, 'hash'], $blacklistedEmails), 1);
diff --git a/mailpoet/lib/Subscription/SubscriptionUrlFactory.php b/mailpoet/lib/Subscription/SubscriptionUrlFactory.php
index 9c09831f2a..28ff9ddd03 100644
--- a/mailpoet/lib/Subscription/SubscriptionUrlFactory.php
+++ b/mailpoet/lib/Subscription/SubscriptionUrlFactory.php
@@ -35,29 +35,29 @@ class SubscriptionUrlFactory {
     $this->linkTokens = $linkTokens;
   }
 
-  public function getConfirmationUrl(SubscriberEntity $subscriber = null) {
+  public function getConfirmationUrl(?SubscriberEntity $subscriber = null) {
     $post = $this->getPost($this->settings->get('subscription.pages.confirmation'));
     return $this->getSubscriptionUrl($post, 'confirm', $subscriber);
   }
 
-  public function getConfirmUnsubscribeUrl(SubscriberEntity $subscriber = null, int $queueId = null) {
+  public function getConfirmUnsubscribeUrl(?SubscriberEntity $subscriber = null, ?int $queueId = null) {
     $post = $this->getPost($this->settings->get('subscription.pages.confirm_unsubscribe'));
     $data = $queueId && $subscriber ? ['queueId' => $queueId] : null;
     return $this->getSubscriptionUrl($post, 'confirm_unsubscribe', $subscriber, $data);
   }
 
-  public function getManageUrl(SubscriberEntity $subscriber = null) {
+  public function getManageUrl(?SubscriberEntity $subscriber = null) {
     $post = $this->getPost($this->settings->get('subscription.pages.manage'));
     return $this->getSubscriptionUrl($post, 'manage', $subscriber);
   }
 
-  public function getUnsubscribeUrl(SubscriberEntity $subscriber = null, int $queueId = null) {
+  public function getUnsubscribeUrl(?SubscriberEntity $subscriber = null, ?int $queueId = null) {
     $post = $this->getPost($this->settings->get('subscription.pages.unsubscribe'));
     $data = $queueId && $subscriber ? ['queueId' => $queueId] : null;
     return $this->getSubscriptionUrl($post, 'unsubscribe', $subscriber, $data);
   }
 
-  public function getReEngagementUrl(SubscriberEntity $subscriber = null) {
+  public function getReEngagementUrl(?SubscriberEntity $subscriber = null) {
     $reEngagementSetting = $this->settings->get('reEngagement');
     $postId = $reEngagementSetting['page'] ?? null;
 
@@ -68,7 +68,7 @@ class SubscriptionUrlFactory {
   public function getSubscriptionUrl(
     $post = null,
     $action = null,
-    SubscriberEntity $subscriber = null,
+    ?SubscriberEntity $subscriber = null,
     $data = null
   ) {
     if ($post === null || $action === null) return;
diff --git a/mailpoet/lib/Twig/Assets.php b/mailpoet/lib/Twig/Assets.php
index b7ec7fd847..28f2233a04 100644
--- a/mailpoet/lib/Twig/Assets.php
+++ b/mailpoet/lib/Twig/Assets.php
@@ -21,7 +21,7 @@ class Assets extends AbstractExtension {
   public function __construct(
     array $globals,
     WPFunctions $wp,
-    CdnAssetUrl $cdnAssetsUrl = null
+    ?CdnAssetUrl $cdnAssetsUrl = null
   ) {
     $this->globals = $globals;
     $this->wp = $wp;
diff --git a/mailpoet/lib/Util/License/Features/Subscribers.php b/mailpoet/lib/Util/License/Features/Subscribers.php
index f0d5f18fb5..1268a04e5d 100644
--- a/mailpoet/lib/Util/License/Features/Subscribers.php
+++ b/mailpoet/lib/Util/License/Features/Subscribers.php
@@ -85,7 +85,7 @@ class Subscribers {
     return $count;
   }
 
-  public function isSubscribersCountEnoughForCache(int $count = null): bool {
+  public function isSubscribersCountEnoughForCache(?int $count = null): bool {
     if (is_null($count) && func_num_args() === 0) {
       $count = $this->getSubscribersCount();
     }
diff --git a/mailpoet/lib/Util/Notices/UnauthorizedEmailNotice.php b/mailpoet/lib/Util/Notices/UnauthorizedEmailNotice.php
index 0fb4bd833a..1975b68a5c 100644
--- a/mailpoet/lib/Util/Notices/UnauthorizedEmailNotice.php
+++ b/mailpoet/lib/Util/Notices/UnauthorizedEmailNotice.php
@@ -20,7 +20,7 @@ class UnauthorizedEmailNotice {
 
   public function __construct(
     WPFunctions $wp,
-    SettingsController $settings = null
+    ?SettingsController $settings = null
   ) {
     $this->settings = $settings;
     $this->wp = $wp;
diff --git a/mailpoet/lib/Validator/Builder.php b/mailpoet/lib/Validator/Builder.php
index d702f626b4..e87be14e33 100644
--- a/mailpoet/lib/Validator/Builder.php
+++ b/mailpoet/lib/Validator/Builder.php
@@ -34,13 +34,13 @@ class Builder {
     return new NullSchema();
   }
 
-  public static function array(Schema $items = null): ArraySchema {
+  public static function array(?Schema $items = null): ArraySchema {
     $array = new ArraySchema();
     return $items ? $array->items($items) : $array;
   }
 
   /** @param array<string, Schema>|null $properties */
-  public static function object(array $properties = null): ObjectSchema {
+  public static function object(?array $properties = null): ObjectSchema {
     $object = new ObjectSchema();
     return $properties === null ? $object : $object->properties($properties);
   }
diff --git a/mailpoet/lib/WP/DateTime.php b/mailpoet/lib/WP/DateTime.php
index e144fcd0e7..f9cf30c269 100644
--- a/mailpoet/lib/WP/DateTime.php
+++ b/mailpoet/lib/WP/DateTime.php
@@ -14,7 +14,7 @@ class DateTime {
   private $wp;
 
   public function __construct(
-    WPFunctions $wp = null
+    ?WPFunctions $wp = null
   ) {
     if ($wp === null) {
       $wp = new WPFunctions();
diff --git a/mailpoet/lib/WP/Emoji.php b/mailpoet/lib/WP/Emoji.php
index 1a6d466d77..016151b7b2 100644
--- a/mailpoet/lib/WP/Emoji.php
+++ b/mailpoet/lib/WP/Emoji.php
@@ -12,7 +12,7 @@ class Emoji {
   private $wp;
 
   public function __construct(
-    WPFunctions $wp = null
+    ?WPFunctions $wp = null
   ) {
     if ($wp === null) {
       $wp = new WPFunctions();
diff --git a/mailpoet/lib/WP/Functions.php b/mailpoet/lib/WP/Functions.php
index c204948ab5..fd51ac1cb8 100644
--- a/mailpoet/lib/WP/Functions.php
+++ b/mailpoet/lib/WP/Functions.php
@@ -296,7 +296,7 @@ class Functions {
     return get_post_type($post);
   }
 
-  public function getPosts(array $args = null) {
+  public function getPosts(?array $args = null) {
     return get_posts($args);
   }
 
@@ -861,7 +861,7 @@ class Functions {
   }
 
   /** @param string[]|null $protocols */
-  public function escUrlRaw(string $url, array $protocols = null): string {
+  public function escUrlRaw(string $url, ?array $protocols = null): string {
     return esc_url_raw($url, $protocols);
   }
 
diff --git a/mailpoet/tasks/release/JiraController.php b/mailpoet/tasks/release/JiraController.php
index fd8d7deed7..0785a2d877 100644
--- a/mailpoet/tasks/release/JiraController.php
+++ b/mailpoet/tasks/release/JiraController.php
@@ -166,7 +166,7 @@ class JiraController {
   /**
    * @see https://developer.atlassian.com/cloud/jira/platform/rest/v3/#api-api-3-search-get
    */
-  public function search($jql, array $fields = null) {
+  public function search($jql, ?array $fields = null) {
     $params = ['jql' => $jql];
     if ($fields) {
       $params['fields'] = join(',', $fields);
diff --git a/mailpoet/tests/DataFactories/Automation.php b/mailpoet/tests/DataFactories/Automation.php
index e15cacc617..cbafa8a1ff 100644
--- a/mailpoet/tests/DataFactories/Automation.php
+++ b/mailpoet/tests/DataFactories/Automation.php
@@ -87,7 +87,7 @@ class Automation {
     return $this->withStep($step);
   }
 
-  public function withIfElseStep(Step $elseStep = null): self {
+  public function withIfElseStep(?Step $elseStep = null): self {
     $subscribedFilter = new Filter(
       'f1',
       'enum',
diff --git a/mailpoet/tests/DataFactories/AutomationRun.php b/mailpoet/tests/DataFactories/AutomationRun.php
index 11fd9b803c..987ecf00b0 100644
--- a/mailpoet/tests/DataFactories/AutomationRun.php
+++ b/mailpoet/tests/DataFactories/AutomationRun.php
@@ -78,7 +78,7 @@ class AutomationRun {
     return $this;
   }
 
-  public function withNextStep(string $nextStep = null): self {
+  public function withNextStep(?string $nextStep = null): self {
     $this->nextStep = $nextStep;
     return $this;
   }
diff --git a/mailpoet/tests/DataFactories/ScheduledTask.php b/mailpoet/tests/DataFactories/ScheduledTask.php
index de28f31816..2716ac6624 100644
--- a/mailpoet/tests/DataFactories/ScheduledTask.php
+++ b/mailpoet/tests/DataFactories/ScheduledTask.php
@@ -35,8 +35,8 @@ class ScheduledTask {
     string $type,
     ?string $status,
     ?\DateTimeInterface $scheduledAt = null,
-    \DateTimeInterface $deletedAt = null,
-    \DateTimeInterface $updatedAt = null
+    ?\DateTimeInterface $deletedAt = null,
+    ?\DateTimeInterface $updatedAt = null
   ) {
     $task = new ScheduledTaskEntity();
     $task->setType($type);
diff --git a/mailpoet/tests/DataFactories/ScheduledTaskSubscriber.php b/mailpoet/tests/DataFactories/ScheduledTaskSubscriber.php
index 92439e8af5..e35413c3b0 100644
--- a/mailpoet/tests/DataFactories/ScheduledTaskSubscriber.php
+++ b/mailpoet/tests/DataFactories/ScheduledTaskSubscriber.php
@@ -31,7 +31,7 @@ class ScheduledTaskSubscriber {
     return $taskSubscriber;
   }
 
-  public function createFailed(ScheduledTaskEntity $task, SubscriberEntity $subscriberEntity, string $error = null): ScheduledTaskSubscriberEntity {
+  public function createFailed(ScheduledTaskEntity $task, SubscriberEntity $subscriberEntity, ?string $error = null): ScheduledTaskSubscriberEntity {
     $taskSubscriber = new ScheduledTaskSubscriberEntity($task, $subscriberEntity, 1, 1, $error);
     $this->entityManager->persist($taskSubscriber);
     $this->entityManager->flush();
diff --git a/mailpoet/tests/DataFactories/SendingQueue.php b/mailpoet/tests/DataFactories/SendingQueue.php
index 1536b44f4b..3de3472e96 100644
--- a/mailpoet/tests/DataFactories/SendingQueue.php
+++ b/mailpoet/tests/DataFactories/SendingQueue.php
@@ -19,7 +19,7 @@ class SendingQueue {
     $this->entityManager = $diContainer->get(EntityManager::class);
   }
 
-  public function create(ScheduledTaskEntity $task, ?NewsletterEntity $newsletter = null, \DateTimeInterface $deletedAt = null): SendingQueueEntity {
+  public function create(ScheduledTaskEntity $task, ?NewsletterEntity $newsletter = null, ?\DateTimeInterface $deletedAt = null): SendingQueueEntity {
     $queue = new SendingQueueEntity();
     $queue->setTask($task);
     $task->setSendingQueue($queue);
diff --git a/mailpoet/tests/DataGenerator/Generators/WooCommercePastRevenues.php b/mailpoet/tests/DataGenerator/Generators/WooCommercePastRevenues.php
index 6eac55102f..f53d795833 100644
--- a/mailpoet/tests/DataGenerator/Generators/WooCommercePastRevenues.php
+++ b/mailpoet/tests/DataGenerator/Generators/WooCommercePastRevenues.php
@@ -510,7 +510,7 @@ class WooCommercePastRevenues implements Generator {
   /**
    * @return \WC_Order|\WP_Error
    */
-  private function createCompletedWooCommerceOrder($subscriberId, $email, $customerId = null, $products = [], Carbon $completedAt = null): \WC_Order {
+  private function createCompletedWooCommerceOrder($subscriberId, $email, $customerId = null, $products = [], ?Carbon $completedAt = null): \WC_Order {
     $random = $this->getRandomString();
     $countries = ['FR', 'GB', 'US', 'IE', 'IT'];
     $address = [
diff --git a/mailpoet/tests/_support/AcceptanceTester.php b/mailpoet/tests/_support/AcceptanceTester.php
index 099ec7ac4d..999d6f950e 100644
--- a/mailpoet/tests/_support/AcceptanceTester.php
+++ b/mailpoet/tests/_support/AcceptanceTester.php
@@ -168,7 +168,7 @@ class AcceptanceTester extends \Codeception\Actor {
     $i->click($xpath);
   }
 
-  public function clickWooTableActionInsideMoreButton(string $itemName, string $moreButtonAction, string $confirmAction = null) {
+  public function clickWooTableActionInsideMoreButton(string $itemName, string $moreButtonAction, ?string $confirmAction = null) {
     $i = $this;
     $i->clickWooTableMoreButtonByItemName($itemName);
     $i->waitForText($itemName);
diff --git a/mailpoet/tests/_support/IntegrationTester.php b/mailpoet/tests/_support/IntegrationTester.php
index 24b5f76640..b5ba4142fd 100644
--- a/mailpoet/tests/_support/IntegrationTester.php
+++ b/mailpoet/tests/_support/IntegrationTester.php
@@ -278,7 +278,7 @@ class IntegrationTester extends \Codeception\Actor {
     return $order;
   }
 
-  public function createWooProductReview(int $customerId, string $customerEmail, int $productId, int $rating, Carbon $date = null): int {
+  public function createWooProductReview(int $customerId, string $customerEmail, int $productId, int $rating, ?Carbon $date = null): int {
     if ($date === null) {
       $date = Carbon::now()->subDay();
     }
diff --git a/mailpoet/tests/acceptance/Automation/AnalyticsCest.php b/mailpoet/tests/acceptance/Automation/AnalyticsCest.php
index 4d4282aaf6..e143c23943 100644
--- a/mailpoet/tests/acceptance/Automation/AnalyticsCest.php
+++ b/mailpoet/tests/acceptance/Automation/AnalyticsCest.php
@@ -244,7 +244,7 @@ class AnalyticsCest {
     return $subscriber;
   }
 
-  private function createRunForSubscriber(SubscriberEntity $subscriber, string $nextStep = null, string $status = AutomationRun::STATUS_COMPLETE) {
+  private function createRunForSubscriber(SubscriberEntity $subscriber, ?string $nextStep = null, string $status = AutomationRun::STATUS_COMPLETE) {
     $run = (new DataFactories\AutomationRun())
       ->withAutomation($this->automation)
       ->withStatus($nextStep ? $status : AutomationRun::STATUS_COMPLETE)
diff --git a/mailpoet/tests/integration/API/JSON/v1/SubscribersTest.php b/mailpoet/tests/integration/API/JSON/v1/SubscribersTest.php
index d66c6665dd..80cdad32ec 100644
--- a/mailpoet/tests/integration/API/JSON/v1/SubscribersTest.php
+++ b/mailpoet/tests/integration/API/JSON/v1/SubscribersTest.php
@@ -314,7 +314,7 @@ class SubscribersTest extends \MailPoetTest {
     verify($response->data)->equals(
       $this->responseBuilder->build($this->subscriber2)
     );
-    verify($this->subscriber2->getSubscriberSegments()->filter(function (SubscriberSegmentEntity $subscriberSegment = null) {
+    verify($this->subscriber2->getSubscriberSegments()->filter(function (?SubscriberSegmentEntity $subscriberSegment = null) {
       if (!$subscriberSegment) return false;
       return $subscriberSegment->getStatus() === SubscriberEntity::STATUS_SUBSCRIBED;
     })->count())->equals(0);
diff --git a/mailpoet/tests/integration/Automation/Engine/Storage/AutomationStatisticsStorageTest.php b/mailpoet/tests/integration/Automation/Engine/Storage/AutomationStatisticsStorageTest.php
index 6b08566598..273d18f71b 100644
--- a/mailpoet/tests/integration/Automation/Engine/Storage/AutomationStatisticsStorageTest.php
+++ b/mailpoet/tests/integration/Automation/Engine/Storage/AutomationStatisticsStorageTest.php
@@ -37,7 +37,7 @@ class AutomationStatisticsStorageTest extends \MailPoetTest {
   /**
    * @dataProvider dataForTestItCalculatesTotalsCorrectly
    */
-  public function testItCalculatesTotalsCorrectlyForSingleAutomation(int $automationIndex, int $expectedTotal, int $expectedInProgress, int $expectedExited, int $versionId = null) {
+  public function testItCalculatesTotalsCorrectlyForSingleAutomation(int $automationIndex, int $expectedTotal, int $expectedInProgress, int $expectedExited, ?int $versionId = null) {
     $automation = $this->automationStorage->getAutomation($this->automations[$automationIndex], $versionId);
     $this->assertInstanceOf(Automation::class, $automation);
     $i = 0;
diff --git a/mailpoet/tests/integration/Automation/Integrations/MailPoet/Fields/NewsletterLinkFieldsTest.php b/mailpoet/tests/integration/Automation/Integrations/MailPoet/Fields/NewsletterLinkFieldsTest.php
index ae92db67f4..432b999238 100644
--- a/mailpoet/tests/integration/Automation/Integrations/MailPoet/Fields/NewsletterLinkFieldsTest.php
+++ b/mailpoet/tests/integration/Automation/Integrations/MailPoet/Fields/NewsletterLinkFieldsTest.php
@@ -50,7 +50,7 @@ class NewsletterLinkFieldsTest extends \MailPoetTest {
     $this->assertEquals($expectedId, $field->getValue($payload));
   }
 
-  private function getPayload(int $id = 1, string $url = 'https://example.com', \DateTimeImmutable $created = null): NewsletterLinkPayload {
+  private function getPayload(int $id = 1, string $url = 'https://example.com', ?\DateTimeImmutable $created = null): NewsletterLinkPayload {
     $newsletter = new NewsletterEntity();
     $queue = new SendingQueueEntity();
     $newsletterLink = new NewsletterLinkEntity($newsletter, $queue, $url, 'hash');
diff --git a/mailpoet/tests/integration/Automation/Integrations/MailPoet/Fields/SubscriberAutomationFieldsFactoryTest.php b/mailpoet/tests/integration/Automation/Integrations/MailPoet/Fields/SubscriberAutomationFieldsFactoryTest.php
index b4b6281d71..0f55065b9b 100644
--- a/mailpoet/tests/integration/Automation/Integrations/MailPoet/Fields/SubscriberAutomationFieldsFactoryTest.php
+++ b/mailpoet/tests/integration/Automation/Integrations/MailPoet/Fields/SubscriberAutomationFieldsFactoryTest.php
@@ -169,7 +169,7 @@ class SubscriberAutomationFieldsFactoryTest extends MailPoetTest {
     Automation $automation,
     string $status,
     array $subjects,
-    DateTimeInterface $createdAt = null
+    ?DateTimeInterface $createdAt = null
   ): AutomationRun {
     $runStorage = $this->diContainer->get(AutomationRunStorage::class);
     $run = $this->tester->createAutomationRun($automation, $subjects);
diff --git a/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysAProductTriggerTest.php b/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysAProductTriggerTest.php
index c5472f1918..13dc96d48f 100644
--- a/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysAProductTriggerTest.php
+++ b/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysAProductTriggerTest.php
@@ -120,7 +120,7 @@ class BuysAProductTriggerTest extends \MailPoetTest {
    * @return \WC_Order
    * @throws \WC_Data_Exception
    */
-  private function createOrder(array $productIds, string $billingEmail = null): \WC_Order {
+  private function createOrder(array $productIds, ?string $billingEmail = null): \WC_Order {
 
     $order = new \WC_Order();
     $order->set_billing_email($billingEmail ?? uniqid() . '@example.com');
diff --git a/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysFromACategoryTriggerTest.php b/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysFromACategoryTriggerTest.php
index 4e95440bee..29209ee8f7 100644
--- a/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysFromACategoryTriggerTest.php
+++ b/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysFromACategoryTriggerTest.php
@@ -136,7 +136,7 @@ class BuysFromACategoryTriggerTest extends \MailPoetTest {
    * @return \WC_Order
    * @throws \WC_Data_Exception
    */
-  private function createOrder(array $productIds, string $billingEmail = null): \WC_Order {
+  private function createOrder(array $productIds, ?string $billingEmail = null): \WC_Order {
 
     $order = new \WC_Order();
     $order->set_billing_email($billingEmail ?? uniqid() . '@example.com');
diff --git a/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysFromATagTriggerTest.php b/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysFromATagTriggerTest.php
index dabc66ee17..9a63daefb2 100644
--- a/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysFromATagTriggerTest.php
+++ b/mailpoet/tests/integration/Automation/Integrations/WooCommerce/Triggers/BuysFromATagTriggerTest.php
@@ -135,7 +135,7 @@ class BuysFromATagTriggerTest extends \MailPoetTest {
    * @return \WC_Order
    * @throws \WC_Data_Exception
    */
-  private function createOrder(array $productIds, string $billingEmail = null): \WC_Order {
+  private function createOrder(array $productIds, ?string $billingEmail = null): \WC_Order {
 
     $order = new \WC_Order();
     $order->set_billing_email($billingEmail ?? uniqid() . '@example.com');
diff --git a/mailpoet/tests/integration/Cron/Triggers/WordPressTest.php b/mailpoet/tests/integration/Cron/Triggers/WordPressTest.php
index 148bc9db4f..3b2949cdb0 100644
--- a/mailpoet/tests/integration/Cron/Triggers/WordPressTest.php
+++ b/mailpoet/tests/integration/Cron/Triggers/WordPressTest.php
@@ -294,7 +294,7 @@ class WordPressTest extends \MailPoetTest {
     verify($this->wordpressTrigger->checkExecutionRequirements())->false();
   }
 
-  private function addMTAConfigAndLog($sent, $status = null, int $retryAt = null) {
+  private function addMTAConfigAndLog($sent, $status = null, ?int $retryAt = null) {
     $mtaConfig = [
       'frequency' => [
         'emails' => 1,
diff --git a/mailpoet/tests/integration/Cron/Workers/KeyCheck/KeyCheckWorkerTest.php b/mailpoet/tests/integration/Cron/Workers/KeyCheck/KeyCheckWorkerTest.php
index ccd40b87b8..cb970a8e59 100644
--- a/mailpoet/tests/integration/Cron/Workers/KeyCheck/KeyCheckWorkerTest.php
+++ b/mailpoet/tests/integration/Cron/Workers/KeyCheck/KeyCheckWorkerTest.php
@@ -106,7 +106,7 @@ class KeyCheckWorkerTest extends \MailPoetTest {
     verify($nextRunDate->diffInSeconds($dateTime))->lessThanOrEqual(21600 + $secondsToMidnight);
   }
 
-  private function createRunningTask(Carbon $scheduledAt = null) {
+  private function createRunningTask(?Carbon $scheduledAt = null) {
     if (!$scheduledAt) {
       $scheduledAt = Carbon::now()->millisecond(0);
     }
diff --git a/mailpoet/tests/integration/Newsletter/Editor/PostContentTransformerTest.php b/mailpoet/tests/integration/Newsletter/Editor/PostContentTransformerTest.php
index 410d164037..32560c6c7f 100644
--- a/mailpoet/tests/integration/Newsletter/Editor/PostContentTransformerTest.php
+++ b/mailpoet/tests/integration/Newsletter/Editor/PostContentTransformerTest.php
@@ -313,7 +313,7 @@ class PostContentTransformerTest extends \MailPoetTest {
   /**
    * @return PostTransformer
    */
-  private function getTransformer(array $args, array $content, array $title, array $image = null, bool $isProduct = false) {
+  private function getTransformer(array $args, array $content, array $title, ?array $image = null, bool $isProduct = false) {
     $extractor = $this->make(
       PostTransformerContentsExtractor::class,
       [
diff --git a/mailpoet/tests/integration/Newsletter/NewsletterSaveControllerTest.php b/mailpoet/tests/integration/Newsletter/NewsletterSaveControllerTest.php
index fe330d0579..9170040080 100644
--- a/mailpoet/tests/integration/Newsletter/NewsletterSaveControllerTest.php
+++ b/mailpoet/tests/integration/Newsletter/NewsletterSaveControllerTest.php
@@ -127,7 +127,7 @@ class NewsletterSaveControllerTest extends \MailPoetTest {
       ],
     ];
     $newsletter = $this->saveController->save($newsletterData);
-    $scheduleOption = $newsletter->getOptions()->filter(function (NewsletterOptionEntity $newsletterOption = null) {
+    $scheduleOption = $newsletter->getOptions()->filter(function (?NewsletterOptionEntity $newsletterOption = null) {
       if ($newsletterOption === null) return false; // PHPStan
       $optionField = $newsletterOption->getOptionField();
       return $optionField && $optionField->getName() === 'schedule';
@@ -140,7 +140,7 @@ class NewsletterSaveControllerTest extends \MailPoetTest {
     $newsletterData['options']['intervalType'] = PostNotificationScheduler::INTERVAL_IMMEDIATELY;
     $savedNewsletter = $this->saveController->save($newsletterData);
 
-    $scheduleOption = $savedNewsletter->getOptions()->filter(function (NewsletterOptionEntity $newsletterOption = null) {
+    $scheduleOption = $savedNewsletter->getOptions()->filter(function (?NewsletterOptionEntity $newsletterOption = null) {
       if ($newsletterOption === null) return false; // PHPStan
       $optionField = $newsletterOption->getOptionField();
       return $optionField && $optionField->getName() === 'schedule';
@@ -184,7 +184,7 @@ class NewsletterSaveControllerTest extends \MailPoetTest {
     ];
 
     $newsletter = $this->saveController->save($newsletterData);
-    $scheduleOption = $newsletter->getOptions()->filter(function (NewsletterOptionEntity $newsletterOption = null) {
+    $scheduleOption = $newsletter->getOptions()->filter(function (?NewsletterOptionEntity $newsletterOption = null) {
       if ($newsletterOption === null) return false; // PHPStan
       $optionField = $newsletterOption->getOptionField();
       return $optionField && $optionField->getName() === 'schedule';
diff --git a/mailpoet/tests/integration/Newsletter/Scheduler/ReEngagementSchedulerTest.php b/mailpoet/tests/integration/Newsletter/Scheduler/ReEngagementSchedulerTest.php
index f1c8e0cd93..4c708e0995 100644
--- a/mailpoet/tests/integration/Newsletter/Scheduler/ReEngagementSchedulerTest.php
+++ b/mailpoet/tests/integration/Newsletter/Scheduler/ReEngagementSchedulerTest.php
@@ -193,7 +193,7 @@ class ReEngagementSchedulerTest extends \MailPoetTest {
     $this->entityManager->flush();
   }
 
-  private function createSubscriber($email, $lastEngagement, SegmentEntity $segment = null) {
+  private function createSubscriber($email, $lastEngagement, ?SegmentEntity $segment = null) {
     $factory = new Subscriber();
     if ($segment) {
       $factory = $factory->withSegments([$segment]);
diff --git a/mailpoet/tests/integration/Newsletter/Sending/SendingQueuesRepositoryTest.php b/mailpoet/tests/integration/Newsletter/Sending/SendingQueuesRepositoryTest.php
index 13a5b7fda4..17dc1f82fb 100644
--- a/mailpoet/tests/integration/Newsletter/Sending/SendingQueuesRepositoryTest.php
+++ b/mailpoet/tests/integration/Newsletter/Sending/SendingQueuesRepositoryTest.php
@@ -148,7 +148,7 @@ class SendingQueuesRepositoryTest extends \MailPoetTest {
     return $task;
   }
 
-  private function createQueue(ScheduledTaskEntity $task, NewsletterEntity $newsletter = null): SendingQueueEntity {
+  private function createQueue(ScheduledTaskEntity $task, ?NewsletterEntity $newsletter = null): SendingQueueEntity {
     if (!$newsletter) {
       $newsletter = new NewsletterEntity();
       $newsletter->setType('type');
diff --git a/mailpoet/tests/integration/Newsletter/ShortcodesTest.php b/mailpoet/tests/integration/Newsletter/ShortcodesTest.php
index 4c22476a3b..fb1102e67a 100644
--- a/mailpoet/tests/integration/Newsletter/ShortcodesTest.php
+++ b/mailpoet/tests/integration/Newsletter/ShortcodesTest.php
@@ -481,7 +481,7 @@ class ShortcodesTest extends \MailPoetTest {
     return $subscriber;
   }
 
-  public function _createNewsletter(NewsletterEntity $parent = null, $type = NewsletterEntity::TYPE_NOTIFICATION): NewsletterEntity {
+  public function _createNewsletter(?NewsletterEntity $parent = null, $type = NewsletterEntity::TYPE_NOTIFICATION): NewsletterEntity {
     $newsletter = new NewsletterEntity();
     $newsletter->setSubject('some subject');
     $newsletter->setType($type);
diff --git a/mailpoet/tests/integration/Newsletter/ViewInBrowser/ViewInBrowserControllerTest.php b/mailpoet/tests/integration/Newsletter/ViewInBrowser/ViewInBrowserControllerTest.php
index 1108e7fb13..b55dc67b07 100644
--- a/mailpoet/tests/integration/Newsletter/ViewInBrowser/ViewInBrowserControllerTest.php
+++ b/mailpoet/tests/integration/Newsletter/ViewInBrowser/ViewInBrowserControllerTest.php
@@ -138,7 +138,7 @@ class ViewInBrowserControllerTest extends \MailPoetTest {
   public function testUsesEmptySubscriberWhenNotLoggedIn() {
 
     $viewInBrowserRenderer = $this->make(ViewInBrowserRenderer::class, [
-      'render' => Expected::once(function (bool $isPreview, NewsletterEntity $newsletter, SubscriberEntity $subscriber = null, SendingQueueEntity $queue = null) {
+      'render' => Expected::once(function (bool $isPreview, NewsletterEntity $newsletter, ?SubscriberEntity $subscriber = null, ?SendingQueueEntity $queue = null) {
         $this->assertNotNull($subscriber); // PHPStan
         verify($subscriber)->notNull();
         verify($subscriber->getId())->equals(0);
@@ -155,7 +155,7 @@ class ViewInBrowserControllerTest extends \MailPoetTest {
   public function testItSetsSubscriberToLoggedInWPUserWhenPreviewIsEnabled() {
     $subscriberId = $this->subscriber->getId();
     $viewInBrowserRenderer = $this->make(ViewInBrowserRenderer::class, [
-      'render' => Expected::once(function (bool $isPreview, NewsletterEntity $newsletter, SubscriberEntity $subscriber = null, SendingQueueEntity $queue = null) use ($subscriberId) {
+      'render' => Expected::once(function (bool $isPreview, NewsletterEntity $newsletter, ?SubscriberEntity $subscriber = null, ?SendingQueueEntity $queue = null) use ($subscriberId) {
         $this->assertNotNull($subscriber); // PHPStan
         verify($subscriber)->notNull();
         verify($subscriber->getId())->equals($subscriberId);
@@ -176,7 +176,7 @@ class ViewInBrowserControllerTest extends \MailPoetTest {
 
   public function testItGetsQueueByQueueId() {
     $viewInBrowserRenderer = $this->make(ViewInBrowserRenderer::class, [
-      'render' => Expected::once(function (bool $isPreview, NewsletterEntity $newsletter, SubscriberEntity $subscriber = null, SendingQueueEntity $queue = null) {
+      'render' => Expected::once(function (bool $isPreview, NewsletterEntity $newsletter, ?SubscriberEntity $subscriber = null, ?SendingQueueEntity $queue = null) {
         $this->assertNotNull($queue); // PHPStan
         verify($queue)->notNull();
         verify($queue->getId())->equals($this->sendingQueue->getId());
@@ -192,7 +192,7 @@ class ViewInBrowserControllerTest extends \MailPoetTest {
 
   public function testItGetsQueueByNewsletter() {
     $viewInBrowserRenderer = $this->make(ViewInBrowserRenderer::class, [
-      'render' => Expected::once(function (bool $isPreview, NewsletterEntity $newsletter, SubscriberEntity $subscriber = null, SendingQueueEntity $queue = null) {
+      'render' => Expected::once(function (bool $isPreview, NewsletterEntity $newsletter, ?SubscriberEntity $subscriber = null, ?SendingQueueEntity $queue = null) {
         $this->assertNotNull($queue); // PHPStan
         verify($queue)->notNull();
         verify($queue->getId())->equals($this->sendingQueue->getId());
diff --git a/mailpoet/tests/integration/REST/API/Endpoint.php b/mailpoet/tests/integration/REST/API/Endpoint.php
index 36f659389f..45bbf491dd 100644
--- a/mailpoet/tests/integration/REST/API/Endpoint.php
+++ b/mailpoet/tests/integration/REST/API/Endpoint.php
@@ -12,7 +12,7 @@ class Endpoint extends APIEndpoint {
   private $requestCallback;
 
   public function __construct(
-    callable $requestCallback = null
+    ?callable $requestCallback = null
   ) {
     $this->requestCallback = $requestCallback;
   }
diff --git a/mailpoet/tests/integration/REST/API/EndpointTest.php b/mailpoet/tests/integration/REST/API/EndpointTest.php
index 8bad4141bf..59c6bb119e 100644
--- a/mailpoet/tests/integration/REST/API/EndpointTest.php
+++ b/mailpoet/tests/integration/REST/API/EndpointTest.php
@@ -107,17 +107,17 @@ class EndpointTest extends Test {
     $this->assertSame($request->getParams(), ['required' => 'required']);
   }
 
-  private function registerTestingGetRoute(string $path, callable $requestCallback = null): void {
+  private function registerTestingGetRoute(string $path, ?callable $requestCallback = null): void {
     $api = $this->createApi($requestCallback);
     $api->registerGetRoute("mailpoet-api-testing-route/$path", Endpoint::class);
   }
 
-  private function registerTestingPostRoute(string $path, callable $requestCallback = null): void {
+  private function registerTestingPostRoute(string $path, ?callable $requestCallback = null): void {
     $api = $this->createApi($requestCallback);
     $api->registerPostRoute("mailpoet-api-testing-route/$path", Endpoint::class);
   }
 
-  private function createApi(callable $requestCallback = null): API {
+  private function createApi(?callable $requestCallback = null): API {
     // ensure REST server is initialized for endpoint registration
     rest_get_server();
 
diff --git a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/EmailActionClickAnyTest.php b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/EmailActionClickAnyTest.php
index b74134667f..44e02d6c52 100644
--- a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/EmailActionClickAnyTest.php
+++ b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/EmailActionClickAnyTest.php
@@ -73,7 +73,7 @@ class EmailActionClickAnyTest extends \MailPoetTest {
     verify($emails[0])->equals('opened_clicked@example.com');
   }
 
-  private function getSegmentFilterData(string $action, int $newsletterId = null, int $linkId = null): DynamicSegmentFilterData {
+  private function getSegmentFilterData(string $action, ?int $newsletterId = null, ?int $linkId = null): DynamicSegmentFilterData {
     return new DynamicSegmentFilterData(DynamicSegmentFilterData::TYPE_EMAIL, $action, [
       'newsletter_id' => $newsletterId,
       'link_id' => $linkId,
diff --git a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/NumberOfClicksTest.php b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/NumberOfClicksTest.php
index 20ed2c4d53..2ae55b712b 100644
--- a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/NumberOfClicksTest.php
+++ b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/NumberOfClicksTest.php
@@ -89,7 +89,7 @@ class NumberOfClicksTest extends \MailPoetTest {
     ]);
   }
 
-  private function createClicks(SubscriberEntity $subscriber, int $count, DateTimeInterface $date = null): void {
+  private function createClicks(SubscriberEntity $subscriber, int $count, ?DateTimeInterface $date = null): void {
     if ($date === null) {
       $date = CarbonImmutable::now();
     }
diff --git a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/UserRoleTest.php b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/UserRoleTest.php
index f5f9ffce16..3ab415adcf 100644
--- a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/UserRoleTest.php
+++ b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/UserRoleTest.php
@@ -82,7 +82,7 @@ class UserRoleTest extends \MailPoetTest {
    * @param string|null $operator
    * @return DynamicSegmentFilterData
    */
-  private function getSegmentFilterData($role, string $operator = null): DynamicSegmentFilterData {
+  private function getSegmentFilterData($role, ?string $operator = null): DynamicSegmentFilterData {
     $filterData = [
       'wordpressRole' => $role,
     ];
diff --git a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceCountryTest.php b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceCountryTest.php
index 6da83a1343..8cbc024a92 100644
--- a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceCountryTest.php
+++ b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceCountryTest.php
@@ -60,7 +60,7 @@ class WooCommerceCountryTest extends \MailPoetTest {
    * @param string|null $operator
    * @return DynamicSegmentFilterData
    */
-  private function getSegmentFilterData($country, string $operator = null): DynamicSegmentFilterData {
+  private function getSegmentFilterData($country, ?string $operator = null): DynamicSegmentFilterData {
     $filterData = [
       'country_code' => $country,
     ];
diff --git a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceSubscriptionTest.php b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceSubscriptionTest.php
index a70ef049b5..6d9b6185fc 100644
--- a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceSubscriptionTest.php
+++ b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceSubscriptionTest.php
@@ -104,7 +104,7 @@ class WooCommerceSubscriptionTest extends \MailPoetTest {
     return (int)$productId;
   }
 
-  private function getSegmentFilterData(string $operator, array $productIds = null): DynamicSegmentFilterData {
+  private function getSegmentFilterData(string $operator, ?array $productIds = null): DynamicSegmentFilterData {
     $filterData = [
       'operator' => $operator,
       'product_ids' => $productIds ?: $this->products,
diff --git a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceUsedCouponCodeTest.php b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceUsedCouponCodeTest.php
index 5344f65856..3acab02afb 100644
--- a/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceUsedCouponCodeTest.php
+++ b/mailpoet/tests/integration/Segments/DynamicSegments/Filters/WooCommerceUsedCouponCodeTest.php
@@ -313,7 +313,7 @@ class WooCommerceUsedCouponCodeTest extends \MailPoetTest {
     $this->assertEqualsCanonicalizing($expectedEmails, $emails);
   }
 
-  private function createOrder(int $customerId, array $couponCodes = [], Carbon $createdAt = null): int {
+  private function createOrder(int $customerId, array $couponCodes = [], ?Carbon $createdAt = null): int {
     if (is_null($createdAt)) {
       $createdAt = Carbon::now()->millisecond(0)->subDay();
     }
diff --git a/mailpoet/tests/integration/Segments/SegmentsFinderTest.php b/mailpoet/tests/integration/Segments/SegmentsFinderTest.php
index 1613c5067c..b1dca6726b 100644
--- a/mailpoet/tests/integration/Segments/SegmentsFinderTest.php
+++ b/mailpoet/tests/integration/Segments/SegmentsFinderTest.php
@@ -94,7 +94,7 @@ class SegmentsFinderTest extends MailPoetTest {
       ->create();
   }
 
-  private function createSegment(string $type, string $name = null): SegmentEntity {
+  private function createSegment(string $type, ?string $name = null): SegmentEntity {
     $name = $name ?? "Segment $type";
     $segmentFactory = new SegmentFactory();
     return $segmentFactory
diff --git a/mailpoet/tests/integration/Segments/WooCommerceTest.php b/mailpoet/tests/integration/Segments/WooCommerceTest.php
index c6ddae8843..b6453a3532 100644
--- a/mailpoet/tests/integration/Segments/WooCommerceTest.php
+++ b/mailpoet/tests/integration/Segments/WooCommerceTest.php
@@ -743,7 +743,7 @@ class WooCommerceTest extends \MailPoetTest {
     return $guest;
   }
 
-  private function insertRegisteredCustomerWithOrder(?int $number = null, array $data = null): WPTestUser {
+  private function insertRegisteredCustomerWithOrder(?int $number = null, ?array $data = null): WPTestUser {
     $number = !is_null($number) ? (int)$number : mt_rand();
     $data = is_array($data) ? $data : [];
     $user = $this->insertRegisteredCustomer($number, $data['first_name'] ?? null, $data['last_name'] ?? null);
diff --git a/mailpoet/tests/integration/Subscribers/ImportExport/PersonalDataExporters/NewsletterClicksExporterTest.php b/mailpoet/tests/integration/Subscribers/ImportExport/PersonalDataExporters/NewsletterClicksExporterTest.php
index 2245f2fe4e..8710a3eefb 100644
--- a/mailpoet/tests/integration/Subscribers/ImportExport/PersonalDataExporters/NewsletterClicksExporterTest.php
+++ b/mailpoet/tests/integration/Subscribers/ImportExport/PersonalDataExporters/NewsletterClicksExporterTest.php
@@ -74,7 +74,7 @@ class NewsletterClicksExporterTest extends \MailPoetTest {
     $this->assertSame($result['data'][0]['data'][3], ['name' => 'User-agent', 'value' => 'Unknown']);
   }
 
-  protected function prepareDataToBeExported(string $userEmail, string $userAgentName = null) {
+  protected function prepareDataToBeExported(string $userEmail, ?string $userAgentName = null) {
     $subscriber = new SubscriberEntity();
     $subscriber->setEmail($userEmail);
     $this->entityManager->persist($subscriber);
diff --git a/mailpoet/tests/integration/Subscribers/ImportExport/PersonalDataExporters/NewsletterOpensExporterTest.php b/mailpoet/tests/integration/Subscribers/ImportExport/PersonalDataExporters/NewsletterOpensExporterTest.php
index 7497fbf9b6..109f95d82b 100644
--- a/mailpoet/tests/integration/Subscribers/ImportExport/PersonalDataExporters/NewsletterOpensExporterTest.php
+++ b/mailpoet/tests/integration/Subscribers/ImportExport/PersonalDataExporters/NewsletterOpensExporterTest.php
@@ -79,7 +79,7 @@ class NewsletterOpensExporterTest extends \MailPoetTest {
     $this->assertSame($result['data'][0]['data'][2], ['name' => 'User-agent', 'value' => 'Unknown']);
   }
 
-  protected function prepareDataToBeExported(string $userEmail, string $userAgentName = null) {
+  protected function prepareDataToBeExported(string $userEmail, ?string $userAgentName = null) {
     $subscriber = new SubscriberEntity();
     $subscriber->setEmail($userEmail);
     $this->entityManager->persist($subscriber);
diff --git a/mailpoet/tests/integration/Subscription/ManageSubscriptionFormRendererTest.php b/mailpoet/tests/integration/Subscription/ManageSubscriptionFormRendererTest.php
index 6689daf52c..30b79544e8 100644
--- a/mailpoet/tests/integration/Subscription/ManageSubscriptionFormRendererTest.php
+++ b/mailpoet/tests/integration/Subscription/ManageSubscriptionFormRendererTest.php
@@ -63,7 +63,7 @@ class ManageSubscriptionFormRendererTest extends \MailPoetTest {
     return $segment;
   }
 
-  private function getSubscriber(SegmentEntity $segment = null): SubscriberEntity {
+  private function getSubscriber(?SegmentEntity $segment = null): SubscriberEntity {
     $subscriber = new SubscriberEntity();
     $subscriber->setStatus(SubscriberEntity::STATUS_SUBSCRIBED);
     $subscriber->setFirstName('Fname');
diff --git a/mailpoet/tests/integration/Subscription/PagesTest.php b/mailpoet/tests/integration/Subscription/PagesTest.php
index 2c05eb77d2..ffff8dd116 100644
--- a/mailpoet/tests/integration/Subscription/PagesTest.php
+++ b/mailpoet/tests/integration/Subscription/PagesTest.php
@@ -295,8 +295,8 @@ class PagesTest extends \MailPoetTest {
   }
 
   private function getPages(
-    NewSubscriberNotificationMailer $newSubscriberNotificationsMock = null,
-    Unsubscribes $unsubscribesMock = null
+    ?NewSubscriberNotificationMailer $newSubscriberNotificationsMock = null,
+    ?Unsubscribes $unsubscribesMock = null
   ): Pages {
     $container = ContainerWrapper::getInstance();
     return new Pages(
diff --git a/mailpoet/tests/unit/Automation/Engine/Validation/AutomationRules/UnknownStepRuleTest.php b/mailpoet/tests/unit/Automation/Engine/Validation/AutomationRules/UnknownStepRuleTest.php
index f424d83244..bc0b650869 100644
--- a/mailpoet/tests/unit/Automation/Engine/Validation/AutomationRules/UnknownStepRuleTest.php
+++ b/mailpoet/tests/unit/Automation/Engine/Validation/AutomationRules/UnknownStepRuleTest.php
@@ -80,7 +80,7 @@ class UnknownStepRuleTest extends AutomationRuleTest {
     (new AutomationWalker())->walk($automation, [$this->getRule($existingAutomation, [new RootStep()])]);
   }
 
-  private function getRule(Automation $existingAutomation = null, array $steps = []): UnknownStepRule {
+  private function getRule(?Automation $existingAutomation = null, array $steps = []): UnknownStepRule {
     $stepMap = [];
     foreach ($steps as $step) {
       $stepMap[$step->getKey()] = $step;
diff --git a/mailpoet/tests/unit/Cron/CronTriggerTest.php b/mailpoet/tests/unit/Cron/CronTriggerTest.php
index 21f5d5bec4..94ae9ae2e2 100644
--- a/mailpoet/tests/unit/Cron/CronTriggerTest.php
+++ b/mailpoet/tests/unit/Cron/CronTriggerTest.php
@@ -62,8 +62,8 @@ class CronTriggerTest extends \MailPoetUnitTest {
 
   private function createCronTrigger(
     SettingsController $settings,
-    WordPress $wordpressTrigger = null,
-    DaemonActionSchedulerRunner $actionSchedulerRunner = null
+    ?WordPress $wordpressTrigger = null,
+    ?DaemonActionSchedulerRunner $actionSchedulerRunner = null
   ) {
     $wordpressTrigger = $wordpressTrigger ?: $this->make(WordPress::class, ['run' => true]);
     $actionSchedulerRunner = $actionSchedulerRunner ?: $this->make(DaemonActionSchedulerRunner::class, ['init' => true]);