Loading old beta or 1.0 worlds crashes the game

The bug

When loading / upgrading worlds which have been created in beta versions or early release versions of Minecraft (released in 2011, maybe also newer versions), the client freezes and has to be terminated through the Windows Task Manager.

occurred for

, the exact version in which the world was created is unknown and some not needed information might have been removed using 3rd party tools, however this also occurs with unmodified worlds, see below.

Opening [^test b1.6.zip] in 20w17a

Worker-10 died
java.util.concurrent.CompletionException: s: Exception generating new chunk
	at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
	at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
	at java.util.concurrent.CompletableFuture.uniCompose(CompletableFuture.java:961)
	at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:926)
	at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442)
	at yf.b(SourceFile:58)
	at yf$$Lambda$3365/1287073396.run(Unknown Source)
	at als.e(SourceFile:83)
	at als.a(SourceFile:123)
	at als.run(SourceFile:91)
	at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1689)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: s: Exception generating new chunk
	at yd.a(SourceFile:579)
	at yd$$Lambda$3367/1964958822.apply(Unknown Source)
	at com.mojang.datafixers.util.Either$Left.map(Either.java:38)
	at yd.a(SourceFile:562)
	at yd$$Lambda$3352/110296568.apply(Unknown Source)
	at java.util.concurrent.CompletableFuture.uniCompose(CompletableFuture.java:952)
	... 12 more
Caused by: java.lang.NullPointerException
	at amg.a(SourceFile:87)
	at amg.<init>(SourceFile:23)
	at yx.d(SourceFile:310)
	at boz.a(SourceFile:342)
	at chz.a(SourceFile:58)
	at cgc.b(SourceFile:102)
	at cgc$$Lambda$1271/23436669.doWork(Unknown Source)
	at cgc$d.doWork(SourceFile:258)
	at cgc.a(SourceFile:214)
	at yd.a(SourceFile:568)
	... 17 more

Opening [^test 1.0.zip] in 20w17a

Error executing task on Chunk source main thread executor for minecraft:overworld
java.util.concurrent.CompletionException: s: Feature placement
	at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
	at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
	at java.util.concurrent.CompletableFuture.uniCompose(CompletableFuture.java:961)
	at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:926)
	at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442)
	at yf.b(SourceFile:58)
	at yf$$Lambda$3355/337692582.run(Unknown Source)
	at als.e(SourceFile:83)
	at als.a(SourceFile:123)
	at als.run(SourceFile:91)
	at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1689)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: s: Feature placement
	at yd.a(SourceFile:579)
	at yd$$Lambda$3357/349989133.apply(Unknown Source)
	at com.mojang.datafixers.util.Either$Left.map(Either.java:38)
	at yd.a(SourceFile:562)
	at yd$$Lambda$3342/455213010.apply(Unknown Source)
	at java.util.concurrent.CompletableFuture.uniCompose(CompletableFuture.java:952)
	... 12 more
Caused by: java.lang.NullPointerException
	at amg.a(SourceFile:87)
	at amg.<init>(SourceFile:23)
	at yx.d(SourceFile:310)
	at csu$a.a(SourceFile:250)
	at ctg.a(SourceFile:76)
	at csu$a.a(SourceFile:268)
	at cte.a(SourceFile:74)
	at clr.a(SourceFile:75)
	at clr$$Lambda$3387/874277545.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at it.unimi.dsi.fastutil.longs.LongIterator$$Lambda$3377/1773788345.accept(Unknown Source)
	at java.util.PrimitiveIterator$OfLong.forEachRemaining(PrimitiveIterator.java:189)
	at it.unimi.dsi.fastutil.longs.LongIterator.forEachRemaining(LongIterator.java:53)
	at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.LongPipeline.reduce(LongPipeline.java:438)
	at java.util.stream.LongPipeline.sum(LongPipeline.java:396)
	at java.util.stream.ReferencePipeline.count(ReferencePipeline.java:526)
	at clr.a(SourceFile:77)
	at cjh.a(SourceFile:54)
	at bps.a(SourceFile:423)
	at cfy.a(SourceFile:132)
	at cgc.c(SourceFile:84)
	at cgc$$Lambda$1271/1271109184.doWork(Unknown Source)
	at cgc.a(SourceFile:214)
	at yd.a(SourceFile:568)
	... 17 more

Reproduction steps

1. Either create a world in Minecraft b1.6 or 1.0, or download

1. or

1. Open the world in the latest version