Issue #2207907 by chx: Comment stubbing is broken.

core/modules/comment/lib/Drupal/comment/Entity/Comment.php

@@ -193,7 +193,9 @@ public static function postDelete(EntityStorageControllerInterface $storage_cont
    */
   public function referencedEntities() {
     $referenced_entities = parent::referencedEntities();
-    $referenced_entities[] = $this->getCommentedEntity();
+    if ($this->getCommentedEntityId()) {
+      $referenced_entities[] = $this->getCommentedEntity();
+    }
     return $referenced_entities;
   }