Exception ticking world entitiesjava.lang.ArrayIndexOutOfBoundsException: -1

While playing on a backup of a survival world which I updated from release 1.13 to snapshot 18w30a, I was casually mining and fought a skeleton. As soon as I began to attack the skeleton, everything slowed down and the game crashed.

Steps to Reproduce:
1. I'm not sure. The circumstances were unlike any crash I've seen before, everything was calm, there was no initial lag or hints that something would go wrong.

Addition Info:

I was using a datapack called "platy" which adds additional advancements to the game.

https://minecraft.curseforge.com/projects/platys-advancement-and-recipe-pack

As far as I'm concerned, this world is dead. I can not seem to recover it so that it will appear on the world select menu. It just, vanished.