protected async override void OnAppearing()
        {
            base.OnAppearing();
            try
            {
                this.Navigation.RemovePage(this.Navigation.NavigationStack[this.Navigation.NavigationStack.Count - 2]);
            }
            catch
            {
                await Navigation.PushAsync(new TeretanePage());
            }

            await model.Init();
        }
 protected async override void OnAppearing()
 {
     base.OnAppearing();
     await model.Init();
 }