App.xaml.cs

public partial class App : Application
{
    public App()
    {
        InitializeComponent();

        NavigationService.Configure("MainPage", typeof(Views.MainPage));
        NavigationService.Configure("ModalNavigationPage", typeof(Views.ModalNavigationPage));
        NavigationService.Configure("PushNavigationPage", typeof(Views.PushNavigationPage));
        var mainPage = ((ViewNavigationService) NavigationService).SetRootPage("MainPage");

        MainPage = mainPage;
    }

    public static INavigationService NavigationService { get; } = new ViewNavigationService();

    // ....
}