unhandled method
      
void
unhandled({ 
    
- _MessageAction<M> ? action,
Indicates that the message has not been handled, and the message should be dispatched to ancestor states for processing.
If action is provided, this action will be invoked before any ancestor states handle the
message. The act builder can be used to specify this action.
Implementation
void unhandled({_MessageAction<M>? action}) {
  _handler = _StayOrUnhandledDescriptor.createForMessage<M>(
    _forState,
    action,
    action?.label,
    _messageName,
    handled: false,
  );
}