World gen datapacks will likely crash or softlock the game

The Bug

In 1.17 Release Candidate 1, creating a world with a datapack will most of the time crash the game, or sometimes give a crash report with a java.lang.ArrayIndexOutOfBoundsException. It may take multiple attempts to reproduce the bug.

How to Reproduce

1. Use the datapack VanillaWorldgen.zip
2. Type in the seed 1 using default world type
3. Load the world

The Error

It will most of the time softlock the game while generating the world, leaving a glitchy screen. Or the other hand, it will crash the game with a crash report, usually like this crash-2021-06-04_11.19.29-server.txt