onChanged property Null safety
Called when the control changes value.
If the control is tapped, onChanged is called immediately with the new value.
The control is considered interactive (see isInteractive) if this callback is non-null. If the callback is null, then the control is disabled, and non-interactive. A disabled checkbox, for example, is displayed using a grey color and its value cannot be changed.
ValueChanged<bool?>? get onChanged;