When loading a 18w20c generated Buffet World (Warm Ocean) in newer snapshot 18w21a, a fatal NullPointerException occurs, causing the game to crash.

To Reproduce

1. Download, extract and place in the saves folder the Minecraft world at http://www.mediafire.com/file/l189qh7bt163aqy/2018-05-23_14-35-40_New_World-.zip/file
2. In the playerdata folder for the save, rename the existing user.dat file to contain your Minecraft UUID (required for issue reproduction)
3. Launch snapshot 18w21a or newer
4. Select Single Player
5. Select the extracted world with your renamed player data file
6. When prompted, click the "I know what I'm doing" button

Notes

• Subsequent loads of the world will not generate error, only the first load of the 18w20c will cause the issue

Crash Report

---- Minecraft Crash Report ----
// I feel sad now

Time: 5/23/18 2:36 PM
Description: Exception ticking world

java.lang.NullPointerException: Exception ticking world
at bwi.a(SourceFile:186)
at bwi.a(SourceFile:92)
at bwm.d(SourceFile:60)
at brt.a(SourceFile:298)
at sj.a(SourceFile:307)
at sk.a(SourceFile:240)
at bbo.a(SourceFile:135)
at sk.i_(SourceFile:199)
at net.minecraft.server.MinecraftServer.w(SourceFile:694)
at net.minecraft.server.MinecraftServer.v(SourceFile:627)
at dii.v(SourceFile:156)
at net.minecraft.server.MinecraftServer.run(SourceFile:532)
at java.lang.Thread.run(Thread.java:745)

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

– Head –
Thread: Server thread
Stacktrace:
at bwi.a(SourceFile:186)
at bwi.a(SourceFile:92)
at bwm.d(SourceFile:60)
at brt.a(SourceFile:298)
at sj.a(SourceFile:307)
at sk.a(SourceFile:240)
at bbo.a(SourceFile:135)
at sk.i_(SourceFile:199)

– Affected level –
Details:
Level name: WaterWorld
All players: 1 total; [sm['kiddailey'/9, l='WaterWorld', x=134.30, y=47.00, z=-560.70]]
Chunk stats: ServerChunkCache: 4068 Drop: 0
Level seed: -7250653221974292753
Level generator: ID 05 - buffet, ver 0. Features enabled: true
Level generator options: {"biome_source":{"type":"minecraft:fixed","options":{"biomes":["minecraft:warm_ocean"]}},"chunk_generator":{"type":"minecraft:surface","options":

Unknown macro: {"default_block"}

}}
Level spawn location: World: (-4,54,-132), Chunk: (at 12,3,12 in -1,-9; contains blocks -16,0,-144 to -1,255,-129), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
Level time: 85714 game time, 85714 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 14416 (now: true), thunder time: 19847 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.w(SourceFile:694)
at net.minecraft.server.MinecraftServer.v(SourceFile:627)
at dii.v(SourceFile:156)
at net.minecraft.server.MinecraftServer.run(SourceFile:532)
at java.lang.Thread.run(Thread.java:745)

– System Details –
Details:
Minecraft Version: 18w21a
Operating System: Windows 8.1 (amd64) version 6.3
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 202386504 bytes (193 MB) / 1023410176 bytes (976 MB) up to 1073741824 bytes (1024 MB)
JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [sm['kiddailey'/9, l='WaterWorld', x=134.30, y=47.00, z=-560.70]]
Data Packs: vanilla
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.