Currently RadComboBox has no property to prevent user from clearing it with backspace. Also ComboBox SelectedIndex may become -1 when ItemSource is changed.
New propery (CanClear) could be checked on KeyDown event, when item collection is changed or when SelectedItem is changed..
If CanClear is set to false:
Backspace is ignored on KeyDown.
If collection is changed and collection size is greater than 0, ComboBox SelectedIndex becomes 0 automatically.
If SelectedItem becomes null and item collection size is greater than 0, ComboBox attemts to select first non-null item from collection.