PopupMenuManager: don't throw exceptions if don't have focus

stage.key_focus can be null, but passing null to _activeMenuContains
throws (from gjs marshalling). In that case, just assume it returned
false.

https://bugzilla.gnome.org/show_bug.cgi?id=642976

js/ui/popupMenu.js

@@ -1252,7 +1252,7 @@ PopupMenuManager.prototype = {
                 menu.sourceActor.grab_key_focus();
         } else if (menu == this._activeMenu) {
             let focus = global.stage.key_focus;
-            let fromActive = this._activeMenuContains(focus);
+            let fromActive = focus && this._activeMenuContains(focus);
 
             if (this.grabbed)
                 this._ungrab();