allow nullable parameters - required for APT Release Lockup Clutch

VECTO3GUI2020/ViewModel/MultiStage/Implementation/InterimStageBusVehicleViewModel_v2_8.cs

@@ -318,6 +318,8 @@ namespace VECTO3GUI2020.ViewModel.MultiStage.Implementation
 			_parameterViewModels[nameof(PredictiveCruiseControlNullable)].EditingChangedCallback = ADASGroupEditingCallback;
 			_parameterViewModels[nameof(ATEcoRollReleaseLockupClutch)].EditingChangedCallback = ADASGroupEditingCallback;
 
+			_parameterViewModels[nameof(ATEcoRollReleaseLockupClutch)].AllowNullValue = true;
+
 			Action<MultistageParameterViewModel> PassengerGroupEditingCallback = (MultistageParameterViewModel param) => {
 				NumberOfPassengersEditingEnabled = param.EditingEnabled;
 			};

VECTO3GUI2020/ViewModel/MultiStage/Implementation/MultistageParameterViewModel.cs

@@ -37,6 +37,7 @@ namespace VECTO3GUI2020.ViewModel.MultiStage.Implementation
 		private object _currentContent;
 		private object _dummyContent;
 		private bool _mandatory;
+		private bool _allowNullValue;
 		private object _storedContent;
 		private string _label;
 		private ObservableCollection<Enum> _allowedItems;
@@ -207,6 +208,16 @@ namespace VECTO3GUI2020.ViewModel.MultiStage.Implementation
 			}
 		}
 
+		public bool AllowNullValue
+		{
+			get => _allowNullValue;
+			set
+			{
+				SetProperty(ref _allowNullValue, value);
+				UpdateValueFieldEditable();
+			}
+		}
+
 		public bool EditingEnabled
 		{
 			get => _editingEnabled;
@@ -398,7 +409,7 @@ namespace VECTO3GUI2020.ViewModel.MultiStage.Implementation
 		/// <returns></returns>
 		public bool UpdateEditingEnabled()
 		{
-			if (CurrentContent == null) {
+			if (!AllowNullValue && CurrentContent == null) {
 				EditingEnabled = false;
 			} else {
 				EditingEnabled = true;