Use $document and don't try to add or remove if the element is not found

Author: BatJan

src/Umbraco.Web.UI.Client/src/common/services/focuslock.service.js

@@ -1,15 +1,19 @@
 (function () {
     "use strict";
 
-    function focusLockService() {
-        var elementToInert = document.querySelector('#mainwrapper');
+    function focusLockService($document) {
+        var elementToInert = $document[0].querySelector('#mainwrapper');
 
         function addInertAttribute() {
-            elementToInert.setAttribute('inert', true);
+            if (elementToInert) {
+                elementToInert.setAttribute('inert', true);
+            }
         }
 
         function removeInertAttribute() {
-            elementToInert.removeAttribute('inert');
+            if (elementToInert) {
+                elementToInert.removeAttribute('inert');
+            }
         }
 
         var service = {
@@ -20,7 +24,7 @@
         return service;
 
     }
-    
+
     angular.module("umbraco.services").factory("focusLockService", focusLockService);
 
 })();