popupMenu: Fix another child popup regression

When switching menus, we need to make sure we close all existing
grabs, not just the first one.

https://bugzilla.gnome.org/show_bug.cgi?id=695859

js/ui/popupMenu.js

@@ -2110,9 +2110,9 @@ const PopupMenuManager = new Lang.Class({
     },
 
     get activeMenu() {
-        let actor = this._grabHelper.currentGrab.actor;
-        if (actor)
-            return actor._delegate;
+        let firstGrab = this._grabHelper.grabStack[0];
+        if (firstGrab)
+            return firstGrab.actor._delegate;
         else
             return null;
     },