Added the MR Form the Patch as per the #9 still needs tests .

Closes #2983605

src/Controller/ActionLinkController.php

@@ -4,14 +4,18 @@ namespace Drupal\flag\Controller;
 
 use Drupal\Component\Utility\Html;
 use Drupal\Core\Ajax\AjaxResponse;
+use Drupal\Core\Ajax\RedirectCommand;
 use Drupal\Core\Ajax\ReplaceCommand;
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Render\RendererInterface;
+use Drupal\Core\Url;
 use Drupal\flag\Ajax\ActionLinkFlashCommand;
 use Drupal\flag\FlagInterface;
 use Drupal\flag\FlagServiceInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\RedirectResponse;
+use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
 /**
@@ -68,7 +72,7 @@ class ActionLinkController implements ContainerInjectionInterface {
    * @param string $view_mode
    *   The flaggable entity view mode.
    *
-   * @return \Drupal\Core\Ajax\AjaxResponse|null
+   * @return \Drupal\Core\Ajax\AjaxResponse|\Symfony\Component\HttpFoundation\RedirectResponse|null
    *   The response object, only if successful.
    *
    * @see \Drupal\flag\Plugin\Reload
@@ -76,20 +80,21 @@ class ActionLinkController implements ContainerInjectionInterface {
   public function flag(FlagInterface $flag, $entity_id, ?string $view_mode = NULL) {
     /** @var \Drupal\Core\Entity\EntityInterface $entity */
     $entity = $this->flagService->getFlaggableById($flag, $entity_id);
-
+    if ($entity != NULL  && $entity instanceof EntityInterface) {
     if ($entity === NULL) {
       throw new NotFoundHttpException();
     }
 
-    try {
-      $this->flagService->flag($flag, $entity);
-    }
-    catch (\LogicException $e) {
-      // Fail silently so we return to the entity, which will show an updated
-      // link for the existing state of the flag.
-    }
+      try {
+        $this->flagService->flag($flag, $entity);
+      } catch (\LogicException $e) {
+        // Fail silently so we return to the entity, which will show an updated
+        // link for the existing state of the flag.
+      }
 
-    return $this->generateResponse($flag, $entity, $flag->getMessage('flag'), $view_mode);
+      return $this->generateResponse($flag, $entity, $flag->getMessage('flag'));
+    }
+    return $this->refreshAjaxReferer();
   }
 
   /**
@@ -110,20 +115,21 @@ class ActionLinkController implements ContainerInjectionInterface {
   public function unflag(FlagInterface $flag, $entity_id, ?string $view_mode = NULL) {
     /** @var \Drupal\Core\Entity\EntityInterface $entity */
     $entity = $this->flagService->getFlaggableById($flag, $entity_id);
-
-    if ($entity === NULL) {
-      throw new NotFoundHttpException();
-    }
-
-    try {
-      $this->flagService->unflag($flag, $entity);
+    if ($entity != NULL && $entity instanceof EntityInterface) {
+
+      if ($entity === NULL) {
+        throw new NotFoundHttpException();
+      }
+      try {
+        $this->flagService->unflag($flag, $entity);
+      } catch (\LogicException $e) {
+        // Fail silently so we return to the entity, which will show an updated
+        // link for the existing state of the flag.
+      }
+
+      return $this->generateResponse($flag, $entity, $flag->getMessage('unflag'));
     }
-    catch (\LogicException $e) {
-      // Fail silently so we return to the entity, which will show an updated
-      // link for the existing state of the flag.
-    }
-
-    return $this->generateResponse($flag, $entity, $flag->getMessage('unflag'), $view_mode);
+    return $this->refreshAjaxReferer();
   }
 
   /**
@@ -165,4 +171,19 @@ class ActionLinkController implements ContainerInjectionInterface {
     return $response;
   }
 
+  /**
+   * Catching the type error if entity is deleted.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse|\Symfony\Component\HttpFoundation\RedirectResponse
+   */
+  private function refreshAjaxReferer(){
+    $referer = \Drupal::request()->server->get('HTTP_REFERER');
+    $fake_request = Request::create($referer);
+    $uri = $fake_request->getRequestUri();
+    $response = new AjaxResponse();
+    $response->addCommand(new RedirectCommand($uri));
+
+    return $response;
+  }
+
 }