java.lang.NullPointerException: mouseClicked event handler error

When I load the mod and then click "create new world" the game crashes, sometimes it does it before that - when I click singleplayer.

This is using the 1.16.5 version of a mod I made

Mcreator version: 2021.1 (although I have also tried the latest 2021.2 snapshot, with the same issue)

Crash report: https://pastebin.com/ifgEDSgV

I have tried regenerating code and removing recently added elements, but I can't find a way to fix it - nothing in the mod is designed to change anything outside of a world, so I have no idea why it would be crashing before I even make one, there's no custom/locked code elements.

1.15.2 and 1.14 versions of the mod are here: https://mcreator.net/modification/64641/shadowlands-new-dimensions-bosses-and-more but those seem to work without crashing, and I have made working 1.16.5 mods before.

I have also tried running Minecraft with MultiMC and the normal launcher and the mod still crashes, so it's not just when I use "run client" in Mcreator

Help would be appreciated :)