Manually applies merge pull request #2556 from dawoe/temp-U4-11167

U4-11167 Fixes rare syncTree error

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

@@ -288,9 +288,12 @@ function navigationService($rootScope, $routeParams, $log, $location, $q, $timeo
                 throw "args.tree cannot be null";
             }
 
-            if (mainTreeEventHandler) {
+            if (mainTreeEventHandler) {                
-                //returns a promise
+               
-                return mainTreeEventHandler.syncTree(args);
+                if (mainTreeEventHandler.syncTree) {
+                    //returns a promise,
+                    return mainTreeEventHandler.syncTree(args);
+                }
             }
 
             //couldn't sync