private void ListViewClick(object sender, ItemTappedEventArgs args){
            MenuItem item = args.Item as MenuItem;
            IsPresented = false;
            new OpenPage().open(item.Id);
        }

public class OpenPage : MasterDetailPage{
    public void open(int id){
        Detail.Navigation.PushAsync(new MyPage());
    }
}

Unhandled Exception:

System.NullReferenceException: Object reference not set to an instance of an object.