Spinoff from this thread on java-user:
If you call addDocument on IndexWriter after it's closed you'll hit a
hard-to-explain NullPointerException (because the RAMDirectory was
closed). Before 2.1, apparently you won't hit any exception and the
IndexWrite will keep running but will have released it's write lock (I
I plan to fix IndexWriter methods to throw an IllegalStateException if
it has been closed.