RadioGroup selected_value is None if the RadioGroup is not on the screen

[s-cork]

Possible Bug
The RadioGroup suffers from the same issue as reported here

https://anvil.works/forum/t/bug-with-radio-button-group-value-on-forms-not-in-page/11404

Which seems a shame

Possible solution:
A RadioGroup only knows about direct children being radio buttons

• breaking change
• easiest to implement
• means you can't put something like a flow panel in a RadioGroup
  • probably means a RadioGroup should support both vertical and horizontal layouts

I added a similar approach to anvil-extras.RadioGroup