Can't get PopToRootAsync working

I start my app showing a main page. From there the user goes through multiple pages until the app should go back to the main page. When I click the final button which should take me back nothing happens. So far I have read, NavigationPage is a must but I use them already.

At the moment I do this:

App.cs:

public App ()
{     
    StartPageView spv = new StartPageView (new StartPageViewModel());
    NavigationPage np = new NavigationPage(spv);
    MainPage = np;
    NavigationUtils.Navigation = MainPage.Navigation;
}

In StartPageViewModel.cs:

    var nextView = new NextView(new NextViewModel());
    NavigationPage np = new NavigationPage(nextView);

    Xamarin.Forms.Device.BeginInvokeOnMainThread(async () =>
    {
        await NavigationUtils.Navigation.PushModalAsync(np);
    }
    );

In NextViewModel.cs:

this.BackCommand = new Command((nothing) =>
    {
        Xamarin.Forms.Device.BeginInvokeOnMainThread(async () =>
        {
            await NavigationUtils.Navigation.PopToRootAsync();
        }
        );
    },
    (nothing) =>
    {
        return true;
    });

When I call the BackCommand in NextViewModel nothing happens. I expect the app to go back to the StartPage.
What do I do wrong?