Ensure the newsletter has hash before preview

[MAILPOET-6273]

mailpoet/lib/API/JSON/v1/Newsletters.php

@@ -28,6 +28,7 @@ use MailPoet\Services\AuthorizedEmailsController;
 use MailPoet\Settings\SettingsController;
 use MailPoet\UnexpectedValueException;
 use MailPoet\Util\License\Features\Subscribers as SubscribersFeature;
+use MailPoet\Util\Security;
 use MailPoet\WP\Emoji;
 use MailPoet\WP\Functions as WPFunctions;
 use MailPoetVendor\Carbon\Carbon;
@@ -327,6 +328,10 @@ class Newsletters extends APIEndpoint {
     $newsletter->setBody(
       json_decode($this->emoji->encodeForUTF8Column($newslettersTableName, 'body', $data['body']), true)
     );
+    // ensure newsletter has hash
+    if (!$newsletter->getHash()) {
+      $newsletter->setHash(Security::generateHash());
+    }
     $this->newslettersRepository->flush();
 
     $response = $this->newslettersResponseBuilder->build($newsletter);