Iam assigning HttpSessionBindingEvent.getSession() to HttpSession variable in a Listener class..
As I understand it, you have created a class implementing HttpSessionBindingListener and attach an instance of this class to each new session. In the valueUnbound method you are trying to get at the session with the Event getSession() method so you can use the session method getLastAccessedTime() - this fails because at this point the session is invalid.
What I am saying is to add a timestamp variable to this custom class - update it everytime you process a request. When the valueUnbound method is called you will have the last access time in the object and wont have to call a session method.
Bill