user join room problem

org.jivesoftware.openfire.muc.spi.MultiUserChatServiceImpl - 内部服务器错误

java.lang.NullPointerException

        at org.jivesoftware.openfire.muc.spi.LocalMUCRoom.sendInitialPresences(LocalMUCRoo m.java:724)

        at org.jivesoftware.openfire.muc.spi.LocalMUCRoom.joinRoom(LocalMUCRoom.java:639)

        at org.jivesoftware.openfire.muc.spi.LocalMUCUser.process(LocalMUCUser.java:485)

        at org.jivesoftware.openfire.muc.spi.LocalMUCUser.process(LocalMUCUser.java:197)

            if (!canAnyoneDiscoverJID() && MUCRole.Role.moderator != joinRole.getRole()) {

                occupantPresence = occupantPresence.createCopy();

                Element frag = occupantPresence.getChildElement("x",

                        "http://jabber.org/protocol/muc#user");

                frag.element("item").addAttribute("jid", null);   //this is 724 line

            }