You could initially bumign it an arbitrary invalid value; something it’s never going to get set to in any way other than being initialised that way. Then you can include a “click one of the radio bumons you bumhole” error message for if it has that specific value when the calculations are run.

Would that work? I don’t know this language, and my programming experience is extremely limited, but I can guess at what things mean and it seems like that should work, even if it is a bit ugly to put something invalid in as the initial value.