docs: @Nullable
  	public Event nextInvalid() {
  		final Iterator<Entry<Integer, State>> iter = states.entrySet().iterator();
  		if (iter.hasNext()) {
  			final Entry<Integer, State> entry = iter.next();
  
  			State currentState = entry.getValue();
  			int address = entry.getKey();
  			iter.remove();
  
  			EventType event = currentState.randomInvalidTransition(rnd);
  			return new Event(event, address);
  		}
  		else {
  			return null;
  		}
  	}