Attempt to get PART_EditableTextBox for RadComboBox fails

[Ken]

I'm currently using the demo version of UI WPF to evaluate the controls, and have placed a RadComboBox into my application by declaring a UserControl of that type in xaml (see attached file).

In the codeback, I then try to get the PART_EditableTextBox, so that I can set some event handlers for it.  This works using the Microsoft ComboBox, but with the RadComboBox, during execution the call to get the TextBox fails, leaving my variable with a value of null.  See attached file.  As you can see, the referenced delegates are defined.  textBox is defined at class level as in the other screenshot.

Why is this happening?

[Ken]

Never mind.  I discovered that the problem was that I defined the RadComboBox as non-editable, so the TextBox is inactive.  Duh!