Get access to NavigationController

Hi

In my AppDelegate i setup the views, but i want to have access to NavigationController... Please advise how i can get the main root view controller / navigation controller?

public override bool FinishedLaunching (UIApplication app, NSDictionary options)
        {
            global::Xamarin.Forms.Forms.Init ();

            MyApplication.PushRegister = this;
            _theApp = new MyApplication ();
            LoadApplication (_theApp);

            var settings = UIUserNotificationSettings.GetSettingsForTypes (UIUserNotificationType.Sound |
                           UIUserNotificationType.Alert | UIUserNotificationType.Badge, null);

            UIApplication.SharedApplication.RegisterUserNotificationSettings (settings);

            return base.FinishedLaunching (app, options);
        }