Session Timeout

Good morining,
In my Application I handle a Session. When I access it at the begin of my Programm, the session is always valid. When I access it later I get sometimes an Invalid session and so a java.lang.IllegalStateException: No SessionManager exception. In debugging are the values of the session null. I didn’t see a posibility to access the session out of an other class. In the UI class I only edit it with get wrapped session. My session-timeout is -1. Why does the session change sometimes it’s value?

Thanks

Daniel