[fix] Avoid JS error when menu is not displayed

Author: nemesifier

openwisp_utils/admin_theme/static/admin/js/menu.js

@@ -13,6 +13,11 @@ const MenuTransitionTime = "0.1s";
 var wasActiveGroupOpen = false;
 
 (function () {
+  // popup page or other pages
+  // where menu is not displayed
+  if (!owMenu) {
+    return;
+  }
   setMenu();
   initGroupViewHandlers();
   initToggleMenuHandlers();