Since the above specified link does not work, I'm giving the following link
http://wiki.sdn.sap.com/wiki/display/XI/Unable+to+convert+Sender+system+to+ALE+logical+system
Basically you have to go to ID and go to adapter specific identifier screen and check whether your idoc adapter has logical business system value. If not set it as the link recommended. Thatswhy you get null pointer exception