Fix error when replying to a post by a deleted user

extensions/mentions/js/forum/src/addPostReplyAction.js

@@ -10,7 +10,8 @@ export default function() {
     if (post.isHidden() || (app.session.user && !post.discussion().canReply())) return;
 
     function insertMention(component, quote) {
-      const mention = '@' + post.user().username() + '#' + post.id() + ' ';
+      const user = post.user();
+      const mention = '@' + (user ? user.username() : post.number()) + '#' + post.id() + ' ';
 
       // If the composer is empty, then assume we're starting a new reply.
       // In which case we don't want the user to have to confirm if they

extensions/mentions/src/Listener/UpdatePostMentionsMetadata.php

@@ -1,4 +1,4 @@
-<?php 
+<?php
 /*
  * This file is part of Flarum.
  *
@@ -129,7 +129,7 @@ class UpdatePostMentionsMetadata
             ->whereIn('id', $mentioned)
             ->get()
             ->filter(function ($post) use ($reply) {
-                return $post->user->id !== $reply->user->id;
+                return $post->user && $post->user->id !== $reply->user_id;
             })
             ->all();