Minecraft crashes with ConcurrentModificationException

Several seconds before the crash I earned the achievement [Librarian], and before that I changed the view distance down to 2 chunks. Not sure if these are related as they occur several seconds prior. I have no clue whether this is repeatable or not. :-/

Attached is the crash report file, along with some context from the log.

P.S. I know corruption is par for the course in dev snapshots, but it seems minecraft doesn't recognize that the map exists anymore, though the directory is still there. Any suggestions?