I keep getting this error: 

Spoiler

java.lang.ArrayIndexOutOfBoundsException: -1
    at it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap.rehash(Long2ObjectOpenHashMap.java:998)
    at it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap.removeEntry(Long2ObjectOpenHashMap.java:280)
    at it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap.remove(Long2ObjectOpenHashMap.java:389)
    at net.minecraft.world.biome.BiomeCache.cleanupCache(BiomeCache.java:73)
    at net.minecraft.world.biome.BiomeProvider.cleanupCache(BiomeProvider.java:237)
    at net.minecraft.world.WorldServer.tick(WorldServer.java:195)
    at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:831)
    at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:743)
    at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:192)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:592)
    at java.lang.Thread.run(Thread.java:748)

With a little bit of Googling I found that it is caused by calling world methods asynchronously. But sadly I cannot fix my issue without knowing which method is causing this, because removing asynchrony from my code would cause tremendous performance problems. I was using world methods asynchronously before and didn't get this error.

Minecraft is pretty much not thread safe. It wasn't written with thread safety in mind and as such can't be treated as if it was. This is but one of the issues you ran into. 

1 hour ago, deerangle said:

I was using world methods asynchronously before and didn't get this error.

If something isn't thread safe and you are using it as if it was it is just a matter of time before you either start running into errors or some weird unintended behaviour emerges.

 

Again:

Minecraft isn't thread safe. You can't assume it is and thus you can't use it as if it was. Stop accessing not thread-safe things asynchronously, that WILL lead to issues because you can't do that.

At best do what you must async and pass what you can't to the main thread to do using IThreadListener#addScheduledTask.

ChunkLoader.getLoadedChunkData(c).thenAccept(data -> Minecraft.getMinecraft().addScheduledTask(() -> ChunkLoader.putChunk(new LoadedChunk(c.x, c.z, c.getWorld(), data))));

So this would work?