FIX: lastPoster not defined correctly in model

app/assets/javascripts/discourse/models/topic.js.es6

@@ -8,7 +8,21 @@ const Topic = RestModel.extend({
   message: null,
   errorLoading: false,
 
-  lastPoster: Ember.computed.alias("posters.lastObject.user"),
+  @computed('posters.firstObject')
+  creator(poster){
+    return poster && poster.user;
+  },
+
+  @computed('posters.lastObject')
+  lastPoster(poster) {
+    if (poster){
+      if (this.last_poster_username === poster.user.username){
+        return poster.user;
+      } else {
+        return this.get('creator');
+      }
+    }
+  },
 
   @computed('fancy_title')
   fancyTitle(title) {