focusColor property Null safety
final
The color for the radio's Material when it has the input focus.
If overlayColor returns a non-null color in the MaterialState.focused state, it will be used instead.
If null, then the value of RadioThemeData.overlayColor is used in the focused state. If that is also null, then the value of ThemeData.focusColor is used.
Implementation
final Color? focusColor;