[BUGFIX] Show error messages for AJAX editing actions in page/list mo…

…dule

In case error occurred (due to missing permission) when editing content
in page or list module, those messages have not been visualized to users.

Resolves: #87971
Releases: 8.7
Change-Id: I331a1e82bc9282a53a4839947fa9cf4d4248b56c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60539
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>

typo3/sysext/backend/Classes/Controller/SimpleDataHandlerController.php

@@ -215,8 +215,8 @@ public function mainAction(ServerRequestInterface $request, ResponseInterface $r
         $this->initClipboard();
         $this->main();
 
-        // Write errors to flash message queue
-        if ($this->prErr) {
+        // Write errors to flash message queue (if not explicitly disabled)
+        if ($this->prErr || $this->prErr === null) {
             $this->tce->printLogErrorMessages();
         }
         if ($this->redirect) {
@@ -249,8 +249,8 @@ public function processAjaxRequest(ServerRequestInterface $request, ResponseInte
             'hasErrors' => false
         ];
 
-        // Prints errors (= write them to the message queue)
-        if ($this->prErr) {
+        // Prints errors (= write them to the message queue, if not explicitly disabled)
+        if ($this->prErr || $this->prErr === null) {
             $content['hasErrors'] = true;
             $this->tce->printLogErrorMessages();
         }