World crashes after changing difficulty: java.util.concurrent.CompletionException: s: Exception generating new chunk

World created in 1.16-pre8, shortly after opening it I changed the difficulty, and the game crashed.

This happened while I was inside of the options menu in singleplayer (no LAN), so the world was paused.

World seed: 6456309125617517717

Stack trace

1.16-pre8\: [^crash-2020-06-18_09.55.34-server.txt]

Description: Exception in server tick loop

java.util.concurrent.CompletionException: s: Exception generating new chunk
	at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314)
	at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:319)
	at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1155)
	at java.base/java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:478)
	at yr.b(SourceFile:58)
	at amq.e(SourceFile:94)
	at amq.a(SourceFile:137)
	at amq.run(SourceFile:105)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1425)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)
Caused by: s: Exception generating new chunk
	at yp.a(SourceFile:579)
	at com.mojang.datafixers.util.Either$Left.map(Either.java:38)
	at yp.a(SourceFile:562)
	at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1146)
	... 11 more
Caused by: java.util.ConcurrentModificationException
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1134)
	at cva.b(SourceFile:64)
	at cva.a(SourceFile:54)
	at com.mojang.datafixers.util.Either$Left.map(Either.java:38)
	at cqc.a(SourceFile:98)
	at cqc.a(SourceFile:131)
	at cpz.a(SourceFile:260)
	at cjs.a(SourceFile:40)
	at cjp$a.a(SourceFile:43)
	at cjp$a.a(SourceFile:34)
	at cml.a(SourceFile:249)
	at ckc.a(SourceFile:27)
	at cha.a(SourceFile:268)
	at cha.a(SourceFile:259)
	at chc.d(SourceFile:51)
	at chc.a(SourceFile:219)
	at yp.a(SourceFile:568)
	... 14 more