How to get UINavigationController from Xamarin.Forms on iOS

I need to get the UINavigationController of a iOS application, but after the method "CreateViewController" of Xamarin.Forms is always null!

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

        window = new UIWindow(UIScreen.MainScreen.Bounds);

        window.RootViewController = App.GetMainPage().CreateViewController();

        window.MakeKeyAndVisible();

        // -->  **window.RootViewController.NavigationController is always NULL!! **
        return true;
    }`

How can I get it?

Thanks.