public async Task OpenModalAsync(Page poPage, bool bNavigationPage = true)
    {

        if (bNavigationPage)
            await Application.Current.MainPage.Navigation.PushModalAsync(new NavigationPage(poPage), true);
        else
            await Application.Current.MainPage.Navigation.PushModalAsync(poPage, true);
    }

NavigationPage.SetHasBackButton(this, true);