add method
Notifies the Bloc of a new event which triggers
all corresponding EventHandler instances.
- 
A StateError will be thrown if there is no event handler registered for the incoming event.
- 
A StateError will be thrown if the bloc is closed and the eventwill not be processed.
Implementation
@override
void add(Event event) {
  if (_sideEffects.isClosed == false) {
    super.add(event);
  } else {
    //Managed issue Bloc trying to add event after closing
  }
}