For some reason, on shutdown of a system, the observerTracker in the SegmentNodeStoreService became null before deactivate() was called, leading to this NPE on line 404:
This in turn lead to the datastore not being closed (4 lines later).
In our case we used the CachingDataStore, which relies on close() being called, if not it triggers
JCR-3873, which prevents the system from starting up again.
In any case, the NPE should be guarded.