As always, thank you very much for the suggestion - keep them coming...
An FRM-30188 at compile time is only a warning and can be ignored assuming it is accurate. Your FMX will still be created.
As for allowing null values in radio buttons, well my initial thought was "hmmm.... maybe". However, after doing some investigation and even some experimenting, this may not be doable or practical. The point of a radio button is to choose one and only one value. If null happens to be one of those possible values then you need to account for it. If you do not allow NULL then there will be no valid value that can be selected and therefore you will not be able to keyboard navigate to the radio group. Why not add a "null" value? For example, it could be presented as "None" or "N/A". Something like this:
If you want toggling behavior then a checkbox is what you need. You can code logic to only allow one checkbox to be selected at a time if necessary.