How to reset pop-up menu display to empty?

I have a pop-up button which initially displays an empty choice. I have an associated procedure which tests what is selected and advises of an incorrect choice (a date field). I want the popup button to then go back to an empty display. I’ve tried setting the associated variable to empty and doing a showwindowvariables command but it doesn’t work. What’s the secret?

OK, I’ve got it to work.