When developing a custom widget, you have to explicitly allow non-persistable entities to be used. Jelte (the widget developer/maintainer) can probably comment further on whether there are any considerations in the widget for using non-persistable entities, but here's how you could change this to try it:
The widget source is on GitHub here.
In the file
src/RadioButtonList/AssocRadioButtonList.xml, you should edit the appropriate property of type "entity" and add this:
EDIT: I logged an issue on GitHub here
A little late to the party... I have added this fix to version 6.3.1 and is updated in the AppStore