fix http://community.sharpdevelop.net/forums/t/13749.aspx

src/AddIns/Misc/SharpRefactoring/Project/Src/Gui/OverrideToStringMethodDialog.xaml.cs

@@ -30,7 +30,7 @@ namespace SharpRefactoring.Gui
 			InitializeComponent();
 			
 			this.baseCall = baseCall;
-			this.fields = fields.Select(f => new PropertyOrFieldWrapper(f) { IsSelected = true }).ToList();
+			this.fields = fields.Where(f => f.ReturnType != null).Select(f => new PropertyOrFieldWrapper(f) { IsSelected = true }).ToList();
 			this.listBox.ItemsSource = this.fields;
 		}
 		

src/AddIns/Misc/SharpRefactoring/Project/Src/Gui/PropertyOrFieldWrapper.cs

@@ -25,7 +25,7 @@ namespace SharpRefactoring.Gui
 		
 		public PropertyOrFieldWrapper(IMember member)
 		{
-			if (member == null || member.ReturnType == null)
+			if (member == null)
 				throw new ArgumentNullException("member");
 			if (!(member is IField || member is IProperty))
 				throw new ArgumentException("member must be IField or IProperty");