less errors in js tests suite

app/assets/javascripts/discourse/components/menu-panel.js.es6

@@ -133,8 +133,7 @@ export default Ember.Component.extend({
       this._resizeInterval = setInterval(() => {
         Ember.run(() => {
           const $panelBodyContents = this.$('.panel-body-contents');
-
-          if ($panelBodyContents.length) {
+          if ($panelBodyContents && $panelBodyContents.length) {
             const contentHeight = parseInt($panelBodyContents.height());
             if (contentHeight !== this._lastHeight) { this.performLayout(); }
             this._lastHeight = contentHeight;