Tried to give me an item from ex nihilo with id exnihilo:item_pebble. Meta for this item goes up to 3, but I've given myself the meta 4, the game crashed and I can no longer enter the world.

Full Crash Report: https://hastebin.com/iderecaxuj.sql

I believe this is a Exnihilo Creatio problem see line 22 of your crash.