private @NotNull ActionCallback addReadyCallback(Object requestor) {
      synchronized (myReadyCallbacks) {
        ActionCallback cb = myReadyCallbacks.get(requestor);
        if (cb == null) {
          cb = new ActionCallback();
          myReadyCallbacks.put(requestor, cb);
        }

        return cb;
      }
    }
private @NotNull ActionCallback addReadyCallback(Object requestor)