Fix another fatal error

extensions/mentions/src/Listener/AddPostMentionedByRelationship.php

@@ -142,10 +142,7 @@ class AddPostMentionedByRelationship
         }
 
         if (isset($posts)) {
-            $posts = array_filter($posts, function ($post) {
-                return is_object($post);
-            });
-            
+            $posts = array_filter((array) $posts, 'is_object');
             $ids = [];
 
             // Once we have the posts, construct a list of the IDs of all of