How to populate a RadComboBox in a widget designer

I'm trying to use a RadComboBox in my widget designer so that the user can make a selection from a list, then I can use that to render the widget with the correct options.

 I've attached screen shots of my widget designer, as well as the code, and as you can see the get_items() method on the RadComboBox keep throwing an error.  Does anyone know how to correct this?