async void onAddBtnClicked(object sender, EventArgs e)
 {
     var nextPage = new ElementoCreateUpdatePage(rubricaUid, categoriaUid, true)
     {
         BindingContext = new Elemento()
     };
     await Navigation.PushAsync(nextPage);
 }
        async public void OnEdit(object sender, EventArgs e)
        {
            var      menuItem = ((MenuItem)sender);
            Elemento elemento = menuItem.CommandParameter as Elemento;

            var nextPage = new ElementoCreateUpdatePage(rubricaUid, categoriaUid, false);

            nextPage.BindingContext = elemento;
            await Navigation.PushAsync(nextPage);
        }