void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
        {
            var todoItem = e.SelectedItem as TodoItem;
            var todoPage = new TodoItemPage();

            todoPage.BindingContext = todoItem;
            Navigation.PushAsync(todoPage);
        }
        void OnAddItemClicked(object sender, EventArgs e)
        {
            var todoItem = new TodoItem()
            {
                ID = Guid.NewGuid().ToString()
            };
            var todoPage = new TodoItemPage(true);

            todoPage.BindingContext = todoItem;
            Navigation.PushAsync(todoPage);
        }