Check whether controlbox exists before assigning

src/converse-muc-views.js

@@ -97,9 +97,7 @@ converse.plugins.add('converse-muc-views', {
         });
 
 
-        Object.assign(_converse.ControlBoxView.prototype, {
-
-            renderRoomsPanel () {
+        function renderRoomsPanel () {
             if (this.roomspanel && u.isVisible(this.roomspanel.el)) {
                 return;
             }
@@ -122,7 +120,9 @@ converse.plugins.add('converse-muc-views', {
              */
             _converse.api.trigger('roomsPanelRendered');
         }
-        });
+        if (_converse.ControlBoxView) {
+            Object.assign(_converse.ControlBoxView.prototype, { renderRoomsPanel });
+        }
 
 
         function ___ (str) {