WINUI3 Desktop: Show Dialog before closing the app

Question

I'm trying to open a ContentDialog when the app is closing to prevent loosing unsafed data. I managed to get the Closing event:

public partial class App : Application
{
public IHost Host
{
    get;
}

public static T GetService<T>() where T : class
{
    if ((App.Current as App)!.Host.Services.GetService(typeof(T)) is not T service)
    {
        throw new ArgumentException($"{typeof(T)} needs to be registered in ConfigureServices within App.xaml.cs.");
    }

    return service;
}

public static WindowEx MainWindow { get; } = new MainWindow();

public static UIElement? AppTitlebar { get; set; }

public App()
{
    InitializeComponent();

    Host = Microsoft.Extensions.Hosting.Host.
    CreateDefaultBuilder().
    UseContentRoot(AppContext.BaseDirectory).
    ConfigureServices((context, services) =>
    {
        // Default Activation Handler
        services.AddTransient<ActivationHandler<LaunchActivatedEventArgs>, DefaultActivationHandler>();

        ...
    }).
    Build();

    UnhandledException += App_UnhandledException;
}

private void App_UnhandledException(object sender, Microsoft.UI.Xaml.UnhandledExceptionEventArgs e)
{
}

protected async override void OnLaunched(LaunchActivatedEventArgs args)
{

    IntPtr hWnd = WinRT.Interop.WindowNative.GetWindowHandle(MainWindow);
    WindowId windowId = Win32Interop.GetWindowIdFromWindow(hWnd);
    AppWindow appWindow = AppWindow.GetFromWindowId(windowId);

    if (appWindow != null)
    {
        appWindow.Closing += AppWindow_Closing;
    }

    base.OnLaunched(args);

    await App.GetService<IActivationService>().ActivateAsync(args);
}

private async void AppWindow_Closing(AppWindow sender, AppWindowClosingEventArgs args)
{
    ContentDialog contentDialog = new ContentDialog()
    {
       Title = "title...",
       Content = "content...",
       CloseButtonText = "Close"
    };
    contentDialog.XamlRoot = MainWindow.Content.XamlRoot;
    await contentDialog.ShowAsync();

    args.Cancel = false; // Set to true => prevents the application to close.
}
}

But the Dialogbox is not showing up. In Debugmode I get an exception: System.Runtime.InteropServoces.COMException

Do you have any ideas how to solve that issue? I already searched the whole day in the net, but coulnd't find a solution