message: fix useEventEmitter
diff: @@ -12,12 +12,14 @@ export class EventEmitter<T> {
  };
  useSubscription = (callback: Subscription<T>) => {
  - const callbackRef = useRef<Subscription<T>>(callback);
  + const callbackRef = useRef<Subscription<T>>();
  callbackRef.current = callback;
  useEffect(() => {
  function subscription(val: T) {
  + if (callbackRef.current) {
  callbackRef.current(val);
  }
  + }
  this.subscriptions.add(subscription);
  return () => {
  this.subscriptions.delete(subscription);
  