I've been seeing this error a lot as i'm flying around in creative single player worlds. I'm still unsure how to reproduce it, so i haven't tested it without other mods yet, or anything.

java.lang.NullPointerException: Unexpected error
at vazkii.psi.common.core.handler.PlayerDataHandler.get(PlayerDataHandler.java:96)
at vazkii.psi.common.core.handler.PlayerDataHandler$EventHandler.onRenderWorldLast(PlayerDataHandler.java:236)

while trying to reproduce this crash, i instead got this:

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
at java.util.HashMap$EntryIterator.next(HashMap.java:1463)
at java.util.HashMap$EntryIterator.next(HashMap.java:1461)
at vazkii.psi.common.core.handler.PlayerDataHandler.cleanup(PlayerDataHandler.java:110)
at vazkii.psi.common.core.handler.PlayerDataHandler$EventHandler.onServerTick(PlayerDataHandler.java:139)

Has this already been reported?

I have the exact same issue. After maybe 3 minutes on one world, my game suddenly crashes.

Oops. Sorry for the many messages; I kinda butterfingered the commit there.