DeliveryAdminWindow: Force user to input gerebelt gewogen input

Elwig/ViewModels/DeliveryAdminViewModel.cs

@@ -116,7 +116,11 @@ namespace Elwig.ViewModels {
         [ObservableProperty]
         private bool _isManualWeighing;
         [ObservableProperty]
-        private bool _isNetWeight;
+        private bool? _isNetWeightValue = false;
+        public bool IsNetWeight {
+            get => IsNetWeightValue ?? false;
+            set => IsNetWeightValue = value;
+        }
 
         [ObservableProperty]
         private WineOrigin? _wineOrigin;