在Xamarin.iOS中设置RootViewController后，NavigationController为null

我在Xamarin.iOS中使用故事板。DashboardViewController（一个UINavigationController）被设置为故事板中的初始视图。但是，在我的AppDelegate类中，FinishedLaunching方法有条件地检查用户在应用程序启动时是否需要登录。如果是这样，它将把"ViewController"（登录控制器）设置为RootViewController，否则它将实例化故事板设置的初始视图控制器，即"DashboardViewController"。这是FinishedLaunching方法的代码。

    public override bool FinishedLaunching (UIApplication application, NSDictionary launchOptions)
    {
        // Override point for customization after application launch.
        // If not required for your application you can safely delete this method
        Window = new UIWindow(UIScreen.MainScreen.Bounds);
        var storyboard = UIStoryboard.FromName("Main", NSBundle.MainBundle);
        bool isLoggedOut = true;
        UIViewController rootViewController;
        if (isLoggedOut)
            rootViewController = (UIViewController)storyboard.InstantiateViewController("ViewController");
        else
            rootViewController = (UIViewController)storyboard.InstantiateInitialViewController();
        Window.RootViewController = rootViewController;
        Window.MakeKeyAndVisible();
    }

一旦验证了用户的凭据，以下代码将尝试将RootViewController再次设置回原始的"DashboardViewController"并显示该视图。这是通过登录控制器完成的。

    partial void LoginBtn_TouchUpInside (UIButton sender)
    {
        var appDelegate = UIApplication.SharedApplication.Delegate as AppDelegate;
        appDelegate.SetRootViewController (dashboardViewController, true);
        this.PresentViewController (dashboardViewController, true, null);
    }

以下是AppDelegate中"SetRootViewController"的代码：

    public void SetRootViewController()
    {
        Window = new UIWindow(UIScreen.MainScreen.Bounds);
        var storyboard = UIStoryboard.FromName("Main", NSBundle.MainBundle);
        bool isLoggedOut = false;
        UIViewController rootViewController;
        if (isLoggedOut)
            rootViewController = (UIViewController)storyboard.InstantiateViewController ("ViewController");
        else 
            rootViewController = (UIViewController)storyboard.InstantiateViewController (viewController);
        Window.RootViewController = rootViewController;
        Window.MakeKeyAndVisible();
    }

到目前为止，这是有效的，但如果我尝试在DashboardViewController（它是一个UINavigationController）中的堆栈中添加另一个视图，通过使用以下代码点击按钮，

    partial void ProfileBtn_TouchUpInside (UIButton sender)
    {
        this.NavigationController.PushViewController (profileViewController, true);
    }

NavigationController为null，应用程序崩溃。因此，此场景的基本流程是"将RootViewController设置为"ViewController"，以便用户可以登录-->当用户登录时，将RootView控制器设置回DashboardViewController并显示该视图-->单击按钮从DashboardView Controller视图导航到另一个视图，但DashboardViewer中的NavigationController为空。

我做错了什么？？如果有任何帮助或建议，我们将不胜感激（我为这个长问题道歉）。

Window.RootViewController = rootViewController;

Window.RootViewController = new UINavigationController(rootViewController);