Game fatally crashed while exploring chunks (IllegalStateException: Accessing PalettedContainer from multiple threads)

The bug

Today I was generating and testing worlds on 21w07a and then I loaded some chunks. But then my game went unresponsive and crashed. This was also happening on a server today too with the worker threads dying.

Stack trace

21w19a\: [^crash-2021-05-18_18.42.52-server.txt]

Description: Accessing PalettedContainer from multiple threads

java.lang.IllegalStateException
	at ahc.a(SourceFile:27)
	at ahc.a(SourceFile:19)
	at cml.a(SourceFile:50)
	at cml.a(SourceFile:109)
	at cmf.a(SourceFile:66)
	at cmf.a(SourceFile:60)
	at cmm.a(SourceFile:179)
	at abr.a(SourceFile:229)
	at bwg.a(SourceFile:15)
	at ctn.c(SourceFile:141)
	at cya.a(SourceFile:75)
	at cxv.a(SourceFile:120)
	at cxv.a(SourceFile:165)
	at cxv.a(SourceFile:92)
	at ctn.a(SourceFile:98)
	at ctn.a(SourceFile:152)
	at cqv.a(SourceFile:55)
	at crb.a(SourceFile:29)
	at java.base/java.util.stream.Streams$StreamBuilderImpl.forEachRemaining(Streams.java:411)
	at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:762)
	at crb.a(SourceFile:28)
	at cqv.a(SourceFile:55)
	at crb.a(SourceFile:29)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625)
	at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:762)
	at crb.a(SourceFile:28)
	at cqv.a(SourceFile:55)
	at crb.a(SourceFile:29)
	at java.base/java.util.stream.Streams$StreamBuilderImpl.forEachRemaining(Streams.java:411)
	at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:762)
	at crb.a(SourceFile:28)
	at cqv.a(SourceFile:55)
	at bwx.a(SourceFile:382)
	at clv.a(SourceFile:245)
	at clx.c(SourceFile:100)
	at clx.a(SourceFile:231)
	at aay.a(SourceFile:563)
	at com.mojang.datafixers.util.Either$Left.map(Either.java:38)
	at aay.a(SourceFile:557)
	at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1146)
	at java.base/java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:478)
	at aba.b(SourceFile:58)
	at aqh.f(SourceFile:92)
	at aqh.a(SourceFile:132)
	at aqh.run(SourceFile:100)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1434)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:295)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1016)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1665)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1598)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:183)