main: Don't mess up the modal stack when the focus actor is destroyed

This seems to be an incorrect conversion when we moved from an array
to an object of keys in 3a6b4f3.

https://bugzilla.gnome.org/show_bug.cgi?id=690965

js/ui/main.js

@@ -553,7 +553,7 @@ function pushModal(actor, params) {
         curFocusDestroyId = curFocus.connect('destroy', function() {
             let index = _findModal(actor);
             if (index >= 0)
-                modalActorFocusStack[index].actor = null;
+                modalActorFocusStack[index].focus = null;
         });
     }
     modalActorFocusStack.push({ actor: actor,