FIX: `topic.details` is not a plain JS Object

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

@@ -554,6 +554,7 @@ const Topic = RestModel.extend({
       }
     }
     keys.forEach(key => this.set(key, json[key]));
+    return this;
   },
 
   reload() {

test/javascripts/components/select-kit/topic-notifications-button-test.js

@@ -5,7 +5,8 @@ import Topic from "discourse/models/topic";
 
 const buildTopic = function(level, archetype = "regular") {
   return Topic.create({
-    id: 4563,
+    id: 4563
+  }).updateFromJson({
     title: "Qunit Test Topic",
     details: {
       notification_level: level