Worlds that have an unknown version now show the latest version

The bug

Worlds that have an unknown version now show the latest version (currently 20w18a).

Opening such a world causes lots of errors to be logged and corrupts the world:

20w18a

Couldn't load chunk [-9, 23]
java.lang.NullPointerException
	at cgo.a(SourceFile:122)
	at cgo$$Lambda$3404/1324669015.accept(Unknown Source)
	at cgu.a(SourceFile:249)
	at cgu$$Lambda$3406/19164340.accept(Unknown Source)
	at it.unimi.dsi.fastutil.ints.Int2IntOpenHashMap$MapEntrySet.forEach(Int2IntOpenHashMap.java:927)
	at cgu.a(SourceFile:249)
	at cgo.h(SourceFile:121)
	at cgz.a(SourceFile:101)
	at yf.l(SourceFile:525)
	at yf$$Lambda$3328/1853807856.get(Unknown Source)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1582)
	at alu.c(SourceFile:144)
	at yr$a.c(SourceFile:540)
	at alu.w(SourceFile:118)
	at yr$a.w(SourceFile:549)
	at yr.d(SourceFile:280)
	at net.minecraft.server.MinecraftServer.bc(SourceFile:777)
	at net.minecraft.server.MinecraftServer.w(SourceFile:765)
	at alu.c(SourceFile:127)
	at net.minecraft.server.MinecraftServer.v(SourceFile:750)
	at net.minecraft.server.MinecraftServer.b(SourceFile:532)
	at net.minecraft.server.MinecraftServer.i_(SourceFile:372)
	at enx.d(SourceFile:65)
	at net.minecraft.server.MinecraftServer.run(SourceFile:673)
	at java.lang.Thread.run(Thread.java:745)

Note: It appears this does not affect worlds in the "Region file format" (before release 1.2.1).

Reproduction steps

1. Start a world in any version before 1.9, or download the attached

1. Copy the world to the saves folder of the current version

2. Open the world selection in the current version