[MC-137746] Crash when generating at exactly the top of the world

The bug

When creating a superflat world with the top block level at exactly y=256 the game crashes during generation or almost immediately after entering the world. The preset 255*minecraft:air,minecraft:stone;minecraft:plains;village is one preset that does this.

Disabling surface structures like villages seems to avoid the problem. Enabling normal decoration like trees and grass also works fine.

I haven't yet tested whether or not this also occurs with the newly re-added customized world type.

Note that layers y=255 or below do work fine (~~MC-136526~~) while layers y=257 or above crash the editor due to ~~MC-68124~~.

Crash report

From 1.13.2

Description: Exception generating new chunk

java.util.concurrent.CompletionException: java.lang.ClassCastException: bcd cannot be cast to ben
	at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
	at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
	at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:604)
	at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:577)
	at java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:443)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: java.lang.ClassCastException: bcd cannot be cast to ben
	at ti.f(SourceFile:196)
	at bxb.a(SourceFile:507)
	at bxb.a(SourceFile:450)
	at bxh$j.a(SourceFile:1764)
	at bxc.a(SourceFile:62)
	at btl.a(SourceFile:78)
	at bvq.a(SourceFile:16)
	at bvq.a(SourceFile:13)
	at bpn.a(SourceFile:27)
	at ayu.a(SourceFile:504)
	at bop.a(SourceFile:183)
	at tr.a(SourceFile:12)
	at tq.a(SourceFile:35)
	at bnd.a(SourceFile:95)
	at tx.a(SourceFile:79)
	at tx.a(SourceFile:25)
	at acu$a.a(SourceFile:147)
	at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:602)
	... 6 more