Fixed NRE in Xwt for DialogButtons

Test code:

```csharp
public class MyDialog : Dialog
{
	public MyDialog ()
	{
		Buttons.Add (new Xwt.DialogButton (Command.Ok) { Sensitive = false });
	}
}
```

The problem being that `ParentDialog` is not yet set by the time `Sensitive = false` is run and, therefore, `ParentDialog.UpdateButton (this);` causes an NRE.
This fix ensures `UpdateButton` is only called when ParentDialog is not null.