Issue #3337216: DI problems

src/Controller/AjaxController.php

@@ -404,7 +404,9 @@ class AjaxController extends ControllerBase implements AjaxControllerInterface {
   protected function loadThread(AjaxResponse $response) {
     $thread_id = $this->requestStack->getCurrentRequest()->get('id');
     if ($thread_id) {
-      $thread = PrivateMessageThread::load($thread_id);
+      $thread = $this->entityTypeManager
+        ->getStorage('private_message_thread')
+        ->load($thread_id);
 
       if ($thread && $thread->access('view', $this->currentUser)) {
         $this->privateMessageService->updateLastCheckTime();