@Nullable
  public BuilderMessageHandler unregisterBuildMessageHandler(UUID sessionId) {
    myCanceledSessions.remove(sessionId);
    final SessionData data = mySessionDescriptors.remove(sessionId);
    if (data == null) {
      return null;
    }
    final Channel channel = data.channel;
    if (channel != null) {
      // cleanup the attribute so that session data is not leaked
      channel.attr(Holder.SESSION_DATA).set(null);
    }
    return data.handler;
  }
@Nullable
  public BuilderMessageHandler unregisterBuildMessageHandler(UUID sessionId)