Null Ref fix

src/AddIns/DisplayBindings/WpfDesign/WpfDesign.XamlDom/Project/XamlProperty.cs

@@ -203,8 +203,10 @@ namespace ICSharpCode.WpfDesign.XamlDom
 			ResetInternal();
 
 			propertyValue = value;
+			if (propertyValue != null) {
 				propertyValue.ParentProperty = this;
 				propertyValue.AddNodeTo(this);
+			}
 			UpdateValueOnInstance();
 
 			ParentObject.OnPropertyChanged(this);