Set to true if this radio list tile is allowed to be returned to an indeterminate state by selecting it again when selected.
To indicate returning to an indeterminate state, onChanged will be called with null.
If false, onChanged will be called with value when it is selected while groupValue != value, and only by selecting another radio button in the group (i.e. changing the value of groupValue) can this radio list tile be unselected.
The default is false.
final bool toggleable;