I'm not sure why your SelectedValue is nothing. In testing this, my SelectedValue seemed to remain at the last selected item when I typed a value that was not in the list.
In your situation though, are you wanting to retrieve the text that the user types? If so, you should be able to do this by accessing the Text property of the RadComboBox.
|If (RadComboBox1.SelectedValue = Nothing) Then
I hope this helps.