I have multiple RadComboBox controls on a page with an empty message. They work great until a user accidentally selects an item and is unable to clear the item so that it shows the empty message again. As soon as the user leaves the control, it shows the last selection they had. I would prefer that a user could press the delete or backspace button and have the selection show the empty message as if they had not made a selection. How do I go about doing that?