The InvitationsMonitors are stored in Map as WeakReference. The getInvitationsMonitor method does not check if get() on the WeakReference returns null.

The according code should be changed to

if (!monitors.containsKey(conn) || monitors.get(conn).get() == null)

Maybe it's also a good idea to change MultiUserChat.InvitationsMonitor.monitors to a Concurrent WeakHashMap.

Downstream bug report: https://code.google.com/p/asmack/issues/detail?id=82