Service threw an exception during method invocation: null錯誤

Java For Flash書中 102頁, 加一行 response.getOutputStream().print("Ok");就可以

  public void service(ServletRequest request, ServletResponse response)
       throws  ServletException, IOException {
                   request.setAttribute("FLASH.RESULT", "TEST Java For Flash!");

                   response.getOutputStream().print("Ok");

  }