I'm aware this is a partial duplicate of #70 .
The solution proposed in that thread should not have been considered a legitimate fix for the issue- it only allows you to edit existing biomes. Biometweaker can do far more than that, so the issue isn't really fixed.

Specifically, biometweaker allows you to register additional biomes. This is something that the NetherEX docs specifically say they can't do. This wouldn't have been much of an issue, assuming that you could add a biome created using biometweaker to the list of biomes that netherEX places during generation. However, when trying to do that, the game crashes when opening a world with a nullPointerException.

As far as I can tell, this is because both biometweaker and netherEX's biome manager fire during onWorldLoad / BIOME_REGISTRY events. NetherEX looks for biometweaker:example_biome and gets a null reference instead. This throws an error and the game crashes.

Perhaps you could delay the check for whether a biome exists until after BIOME_REGISTRY had finished?

Edit: crashlog attached.
https://mclo.gs/UH68cVw

Update: Perhaps the warning at the top of the reddit page saying "the docs are outdated" might have meant something. Using a script like this (https://gist.github.com/RedRaspberry/abea50569b3d4c14275072425a95393f) does allow the biome to spawn in world; however you have to ensure that the filler block (and ActualFillerBlock) are NOT set in biometweaker , and only do the filler, top block, etc, spawning in this json. A bit ugly, but it does work.
Basically, you have to ONLY use biometweaker for biome registry, and then use netherEX for... everything else. Which has the flaw of there being no documentation of how the 1.12 json system works, since the wiki linked in #70 is for an older version.

If anyone still maintains this mod (I know, it's quite old and for 1.12, so I'm not really expecting it), do you know of documentation for the 1.12 json format that I'm just missing? I'd rather not have to reverse-engineer it myself anymore than I already have.