Crash: java.lang.NullPointerException: Exception ticking world

The game is crashed when playing.

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:249)
	at bbo.a(SourceFile:144)
	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)

_{Mod edit: Copied over from MC-130132:}

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