First of all having everything in session is a big waste of space - makes your development easy but it will bite you in the long run - as the app scales, more users log in - there is only so much of server memory.
Why is it NPEing? - looking at the stacktrace will give you a good idea - my initial guess is because of your code/app - also try debugging the app - so you can inspect and see where you are trying to use a null object - mostly a != null before using that object will make the problem go away- but I will look more closely to see why such a thing happened in the first place.
Good luck !! - also pasting relevant stack trace will help - but do not paste the 42 page stack trace...no one here has so much patience/time/sanity to look at such a thing.