Weird NullPointer in Biome Colors

I've been experiencing this weird NPE crash for the last couple of weeks or so, while working on a server which does some custom terrain generation.

This crash occurs in both vanilla and modded (optifine, forge) versions of 1.13.2. The server mentioned also uses [ViaVersion|https://www.spigotmc.org/resources/viaversion.19254/] to support connections with 1.14. This is not the cause of the crash though, as I have also tested this without it, but it also catches a similar issue in the ChunkData packet, running out if ints to read here: https://github.com/ViaVersion/ViaVersion/blob/master/common/src/main/java/us/myles/ViaVersion/protocols/protocol1_13to1_12_2/types/Chunk1_13Type.java#L58   suggesting a null element or something in the biome chunk data.

 

According to the attached crash logs, the Exception occurs in the BiomeColors.class in the #getColor method. I've been trying to debug this for the past few days now, and I am a bit confused as to how this NPE can even occur. So for the stuff I figured out about the affected line:

• #getColor of a BiomeColors$ColorResolver is called -> resolver can't be null as it is always a static variable passed in by one of the public methods in that class
• #getBiome is called on the IWorldReaderBase argument -> can't be null either, since the BlockColors.class does a !=null check before even calling any of the BiomeColors methods (same goes for the BlockPos argument)
• I tried overriding the GRASS_COLOR ColorResolver and noticed that the first argument (biome) can be null occasionally, so that might be the issue. Though there's nothing on the server that should be able to cause a null Biome. Also, looking at the decompiled source, the NPE should occur in the Biome.class in #getTemperature, trying to get the y value of the possibly null BlockPos (maybe some decompiler weirdness, idk)

While digging around I also noticed that the WorldGenRegion.class' #getBiome method has actually checks if the returned biome is null and throws a RuntimeException, while other classes implementing that method don't. 

 

 

As for the server (using PaperSpigot) I've also been digging around a bit and wasn't able to find anything that would cause a null Biome or empty array element in the ChunkData packet.