Check type for PHPStan

[MAILPOET-2535]

lib/Util/pQuery/pQuery.php

@@ -6,6 +6,12 @@ namespace MailPoet\Util\pQuery;
 class pQuery extends \pQuery {
   public static function parseStr($html) {
     $parser = new Html5Parser($html);
+
+    if (!$parser->root instanceof \pQuery\DomNode) {
+      // this condition shouldn't happen it is here only for PHPStan
+      throw new \Exception('Renderer is not configured correctly');
+    }
+
     return $parser->root;
   }
 }