How do I access my child viewcontroller instance variable from my navigationcontroller in order to save user settings upon iphone app termination?

Question

I'm trying to save a couple of user settings upon iphone app termination. In my AppDelegate applicationWillTerminate, I reference the child viewcontroller as follows:

ChildViewController *childvc = (ChildViewController *) self.navController.topViewController;

as the instance variable I'm after is in the top viewcontroller. However, I childvc seems to be blank. Is this the right approach or is there another way to retrieve child viewcontroller variables back to the AppDelegate?