[MC-126241] Crash on World Load - Skyblock 2.1 Vanilla

The bug

Upon opening a freshly unzipped Skyblock 2.1 world in 18w08b, the MC client hangs at "Loading world".

latest.log contains lines referencing several "java.lang.IllegalArgumentException: bound must be positive" errors.

Notes

The world file is initially in old Beta 1.8 format and must undergo conversion.
This conversion cannot happen in 18w08b; it insists on recreating the world, even if you click "I know what I'm doing" – possible 2nd bug candidate there.
Conversion of the world performed under release 1.12.2, where it loads just fine.

Crash report

18w08b: crash-2018-02-23_14.34.41-server.txt

Description: Exception in server tick loop

java.lang.RuntimeException: java.util.concurrent.ExecutionException: java.lang.IllegalArgumentException: bound must be positive
	at net.minecraft.server.MinecraftServer.i_(SourceFile:394)
	at cxd.a(SourceFile:115)
	at cxd.d(SourceFile:131)
	at net.minecraft.server.MinecraftServer.run(SourceFile:502)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.util.concurrent.ExecutionException: java.lang.IllegalArgumentException: bound must be positive
	at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:2272)
	at net.minecraft.server.MinecraftServer.i_(SourceFile:392)
	... 4 more
Caused by: java.lang.IllegalArgumentException: bound must be positive
	at java.util.Random.nextInt(Random.java:388)
	at bob.a(SourceFile:19)
	at bob.a(SourceFile:12)
	at bjq.a(SourceFile:27)
	at aun.a(SourceFile:457)
	at bjb.a(SourceFile:123)
	at sk.a(SourceFile:12)
	at sj.a(SourceFile:33)
	at bhn.a(SourceFile:85)
	at sp.a(SourceFile:54)
	at sp.a(SourceFile:23)
	at zl$a.a(SourceFile:143)
	at zl$a$$Lambda$917/475551630.apply(Unknown Source)
	at java.util.concurrent.CompletableFuture$AsyncApply.exec(CompletableFuture.java:501)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:902)
	at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1689)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1644)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)