private async void SaveButton_Click(object sender, RoutedEventArgs e)
        {
            SaveButton.IsEnabled   = false;
            CancelButton.IsEnabled = false;
            var trainingsService = new TrainingsService(App.__APIKey);
            var success          = await trainingsService.SaveTrainingAsync(_edit);

            if (success.Erfolgreich)
            {
                this.Frame.GoBack();
            }
            else
            {
                Windows.UI.Popups.MessageDialog dialog = new Windows.UI.Popups.MessageDialog(success.Fehlermeldung);
                await dialog.ShowAsync();
            }
            SaveButton.IsEnabled   = true;
            CancelButton.IsEnabled = true;
        }