DeliveryAdminWindow: Unselect wine quality level on invalid gradation

Elwig/Windows/DeliveryAdminWindow.xaml.cs

@@ -1236,6 +1236,7 @@ namespace Elwig.Windows {
                 UnsetDefaultValue(WineQualityLevelInput);
                 ComboBox_SelectionChanged(WineQualityLevelInput, null);
                 WineQualityLevelInput.ItemsSource = ctx.WineQualityLevels.ToList();
+                WineQualityLevelInput.SelectedItem = null;
                 return;
             }
             var kmw = (double)ViewModel.GradationKmw!;