[Locator] Don't reset root unnecessarily

...as this will trigger a refresh of the mct-representation for
the tree, which will in turn create a new mct-tree instance,
resulting in any expanded/collapsed state being lost.

Fixes #1008.

platform/commonUI/edit/src/creation/LocatorController.js

@@ -50,7 +50,7 @@ define(
                         $scope.rootObject =
                             (context && context.getRoot()) || $scope.rootObject;
                     }, 0);
-                } else if (!contextRoot) {
+                } else if (!contextRoot && !$scope.rootObject) {
                     //If no context root is available, default to the root
                     // object
                     $scope.rootObject = undefined;