FIX: Category permissions were broken

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

@@ -1,19 +1,24 @@
 import RestModel from 'discourse/models/rest';
+import { on } from 'ember-addons/ember-computed-decorators';
 
 const Category = RestModel.extend({
 
-  init: function() {
-    this._super();
-    var availableGroups = Em.A(this.get("available_groups"));
-
+  @on('init')
+  setupGroupsAndPermissions() {
+    const availableGroups = this.get('available_groups');
+    if (!availableGroups) { return; }
     this.set("availableGroups", availableGroups);
-    this.set("permissions", Em.A(_.map(this.group_permissions, function(elem){
+
+    const groupPermissions = this.get('group_permissions');
+    if (groupPermissions) {
+      this.set('permissions', groupPermissions.map((elem) => {
         availableGroups.removeObject(elem.group_name);
         return {
           group_name: elem.group_name,
           permission: Discourse.PermissionType.create({id: elem.permission_type})
         };
-    })));
+      }));
+    }
   },
 
   availablePermissions: function(){

app/assets/javascripts/discourse/routes/application.js.es6

@@ -129,6 +129,7 @@ const ApplicationRoute = Discourse.Route.extend(OpenComposer, {
     editCategory(category) {
       Category.reloadById(category.get('id')).then((atts) => {
         const model = this.store.createRecord('category', atts.category);
+        model.setupGroupsAndPermissions();
         this.site.updateCategory(model);
         showModal('editCategory', { model });
         this.controllerFor('editCategory').set('selectedTab', 'general');