Don't display notifications with deleted subjects

framework/core/js/forum/src/components/NotificationList.js

@@ -33,12 +33,14 @@ export default class NotificationList extends Component {
       app.cache.notifications.forEach(notification => {
         const subject = notification.subject();
 
+        if (typeof subject === 'undefined') return;
+
         // Get the discussion that this notification is related to. If it's not
         // directly related to a discussion, it may be related to a post or
         // other entity which is related to a discussion.
-        let discussion;
+        let discussion = false;
         if (subject instanceof Discussion) discussion = subject;
-        else if (subject.discussion) discussion = subject.discussion();
+        else if (subject && subject.discussion) discussion = subject.discussion();
 
         // If the notification is not related to a discussion directly or
         // indirectly, then we will assign it to a neutral group.

framework/core/js/forum/src/components/TerminalPost.js

@@ -1,6 +1,5 @@
 import Component from 'flarum/Component';
 import humanTime from 'flarum/helpers/humanTime';
-import username from 'flarum/helpers/username';
 
 /**
  * Displays information about a the first or last post in a discussion.

framework/core/js/lib/Model.js

@@ -227,6 +227,8 @@ export default class Model {
 
         return relationship && app.store.getById(relationship.data.type, relationship.data.id);
       }
+
+      return false;
     };
   }
 
@@ -247,6 +249,8 @@ export default class Model {
 
         return relationship && relationship.data.map(data => app.store.getById(data.type, data.id));
       }
+
+      return false;
     };
   }
 

framework/core/src/Api/Serializers/Serializer.php

@@ -115,7 +115,8 @@ abstract class Serializer extends SerializerAbstract
                 } elseif ($many) {
                     $relationIds = $relation.'_ids';
                     $data = isset($model->$relationIds) ? $model->$relationIds : $model->$relation()->lists('id');
-                } else {
+                }
+                if (! $many && empty($data)) {
                     $relationId = $relation.'_id';
                     $data = $model->$relationId;
                 }