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.
[media]occurred for
[media], 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
Either create a world in Minecraft b1.6 or 1.0, or download
or
Open the world in the latest version
Attachments
Comments


Uploaded .zip world file. Also, I tried playing it with 20w16a and I have no problems, I've been playing it for 2-3 days now. 20w17a didn't gave me the "old world" warning, but 20w16a did.

I was able to reproduce this bug, but I am not sure if the world you attached (
[media]) is valid, since it appears some information might have been removed from it using third party tools.
Nonetheless I was also able to reproduce this by creating worlds in these versions myself.
I have edited the report, I hope this is alright for you.

The world is completely valid, since I opened the world with 20w16a and had no problems. I also opened it with 1.15.2, 1.15.1, 1.14.4, 1.14, 1.13.2, and below and had no issues.

Can confirm fixed. Worlds can be opened in 20w19a, however all chunks are reset, which might be MC-182514.
Could you please provide a link to the world (before you tried opening it in 20w17a) or attach it here as zip?