In at.feldim2425.moreoverlays.lightoverlay.integration.CustomSpawnerLightScanner

    List<Biome.SpawnListEntry> possibleSpawns = CustomSpawner.instance().getPossibleCustomCreatures(world, environment.entitySpawnTypes.get("MONSTER"), pos.getX(), pos.getY(), pos.getZ());

    return !possibleSpawns.isEmpty();

The returned list possibleSpawns can be null when changing worlds (i.E. to the Nether or back), which will then instantly result in a crash. This can also happen when logging in, which then causes the world to be unplayable until MoreOverlays is deactivated.

Using Minecraft 1.12 with Forge 2854. MoreOverlays 21.7.19, CustomMobSpawner 3.11.5.

This is a duplicate of #72