Custom dimension with custom biomes causes crash on load

Hey, all!

I'm here to report a bug and/or hopefully get a little bit of help with it. I'm trying to make a dimension with some custom biomes, but when I attempt to load in it crashes. Part of the issue is that what causes it to crash is a complete mystery to me - if I only set the dimension to use two of my custom biomes then it's fine, and sometimes I can even get it to use up to four, but after a few attempts to start a world and generate the dimension it will eventually crash a few seconds after I enter, presumably while it's trying to load the chunks around where the portal spawned. The weird thing is that once this happens, that's pretty much the ball game for those biomes in that dimension, because no matter how I change it afterwards (using fewer biomes, changing the generation settings, using entirely different biomes, etc.) the dimension will permanently be broken so long as I use whatever biomes I had in there when it crashed in the dimension. If I make brand new biomes it'll work fine, until of course I make some changes. The last time it happened (the crash the upcoming crash report is from) I didn't even change the dimension's biomes - I did make new biomes, but I forgot to add them, and even though on the last test it loaded just fine and I could explore as much as I wanted with only two biomes, when I loaded it up the next time it crashed upon spawning.

Does anyone have any clue as to what this could be caused by? I've made another dimension using vanilla biomes and it seems to work just fine, so I suspect that custom biomes seem to be part of the problem. Here's the crash report:

---- Minecraft Crash Report ----
// Don't do that.

Time: 9/5/20 12:35 AM
Description: Exception ticking world

java.lang.IllegalArgumentException: null
    at net.minecraft.util.WeightedRandom.getRandomItem(WeightedRandom.java:27) ~[?:?] {re:classloading}
    at net.minecraft.util.WeightedRandom.getRandomItem(WeightedRandom.java:52) ~[?:?] {re:classloading}
    at net.minecraft.world.spawner.WorldEntitySpawner.getSpawnList(WorldEntitySpawner.java:162) ~[?:?] {re:classloading}
    at net.minecraft.world.spawner.WorldEntitySpawner.spawnEntitiesInChunk(WorldEntitySpawner.java:90) ~[?:?] {re:classloading}
    at net.minecraft.world.server.ServerChunkProvider.lambda$tickChunks$5(ServerChunkProvider.java:372) ~[?:?] {re:classloading,pl:accesstransformer:B}
    at it.unimi.dsi.fastutil.longs.Long2ObjectLinkedOpenHashMap$1.forEach(Long2ObjectLinkedOpenHashMap.java:1661) ~[fastutil-8.2.1.jar:?] {}
    at com.google.common.collect.Iterables$UnmodifiableIterable.forEach(Iterables.java:105) ~[guava-21.0.jar:?] {}
    at net.minecraft.world.server.ServerChunkProvider.tickChunks(ServerChunkProvider.java:355) ~[?:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.world.server.ServerChunkProvider.tick(ServerChunkProvider.java:331) ~[?:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.world.server.ServerWorld.tick(ServerWorld.java:316) ~[?:?] {re:classloading}
    at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:886) ~[?:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:821) ~[?:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:120) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:662) [?:?] {re:classloading,pl:accesstransformer:B}
    at java.lang.Thread.run(Thread.java:748) [?:1.8.0_252] {}

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Server thread
Stacktrace:
    at net.minecraft.util.WeightedRandom.getRandomItem(WeightedRandom.java:27)
    at net.minecraft.util.WeightedRandom.getRandomItem(WeightedRandom.java:52)
    at net.minecraft.world.spawner.WorldEntitySpawner.getSpawnList(WorldEntitySpawner.java:162)
    at net.minecraft.world.spawner.WorldEntitySpawner.spawnEntitiesInChunk(WorldEntitySpawner.java:90)
    at net.minecraft.world.server.ServerChunkProvider.lambda$tickChunks$5(ServerChunkProvider.java:372)
    at it.unimi.dsi.fastutil.longs.Long2ObjectLinkedOpenHashMap$1.forEach(Long2ObjectLinkedOpenHashMap.java:1661)
    at com.google.common.collect.Iterables$UnmodifiableIterable.forEach(Iterables.java:105)
    at net.minecraft.world.server.ServerChunkProvider.tickChunks(ServerChunkProvider.java:355)
    at net.minecraft.world.server.ServerChunkProvider.tick(ServerChunkProvider.java:331)
    at net.minecraft.world.server.ServerWorld.tick(ServerWorld.java:316)

-- Affected level --
Details:
    All players: 1 total; [ServerPlayerEntity['Dev'/276, l='HOZIER?', x=-216.50, y=36.02, z=-176.25]]
    Chunk stats: ServerChunkCache: 868
    Level dimension: DimensionType{dungeons_and_dragons:feywild}
    Derived: true
    Level name: HOZIER?
    Level seed: 1736655141481778848
    Level generator: ID 00 - default, ver 1. Features enabled: true
    Level generator options: {}
    Level spawn location: World: (-212,63,-192), Chunk: (at 12,3,0 in -14,-12; contains blocks -224,0,-192 to -209,255,-177), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
    Level time: 857 game time, 857 day time
    Known server brands: forge
    Level was modded: true
    Level storage version: 0x04ABD - Anvil
    Level weather: Rain time: 152487 (now: false), thunder time: 35883 (now: false)
    Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
    at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:886)
    at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:821)
    at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:120)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:662)
    at java.lang.Thread.run(Thread.java:748)

-- System Details --
Details:
    Minecraft Version: 1.15.2
    Minecraft Version ID: 1.15.2
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 1.8.0_252, BellSoft
    Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), BellSoft
    Memory: 997716264 bytes (951 MB) / 2740453376 bytes (2613 MB) up to 5715787776 bytes (5451 MB)
    CPUs: 12
    JVM Flags: 1 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump
    ModLauncher: 5.1.2+70+master.2845bb9
    ModLauncher launch target: fmluserdevclient
    ModLauncher naming: mcp
    ModLauncher services: 
        /eventbus-2.2.0-service.jar eventbus PLUGINSERVICE 
        /forge-1.15.2-31.2.0_mapped_snapshot_20200514-1.15.1-launcher.jar object_holder_definalize PLUGINSERVICE 
        /forge-1.15.2-31.2.0_mapped_snapshot_20200514-1.15.1-launcher.jar runtime_enum_extender PLUGINSERVICE 
        /accesstransformers-2.1.1-shadowed.jar accesstransformer PLUGINSERVICE 
        /forge-1.15.2-31.2.0_mapped_snapshot_20200514-1.15.1-launcher.jar capability_inject_definalize PLUGINSERVICE 
        /forge-1.15.2-31.2.0_mapped_snapshot_20200514-1.15.1-launcher.jar runtimedistcleaner PLUGINSERVICE 
        /forge-1.15.2-31.2.0_mapped_snapshot_20200514-1.15.1-launcher.jar fml TRANSFORMATIONSERVICE 
    FML: 31.2
    Forge: net.minecraftforge:31.2.0
    FML Language Providers: 
        javafml@31.2
        minecraft@1
    Mod List: 
        client-extra.jar Minecraft {minecraft@1.15.2 DONE}
        main Dungeons and Dragons {dungeons_and_dragons@0.0.11 DONE}
        forge-1.15.2-31.2.0_mapped_snapshot_20200514-1.15.1-recomp.jar Forge {forge@31.2.0 DONE}
    Player Count: 1 / 8; [ServerPlayerEntity['Dev'/276, l='HOZIER?', x=-216.50, y=36.02, z=-176.25]]
    Data Packs: vanilla, mod:dungeons_and_dragons, mod:forge (incompatible)
    Type: Integrated Server (map_client.txt)
    Is Modded: Definitely; Client brand changed to 'forge'