NullPointerException in server tick loop when trying to load a flatland world with Nether biome

The bug

I wanted to load the world from the description of this video in 20w09a: https://www.youtube.com/watch?v=l9oteFEj770

However, when I tried to open it, the game crashed.

1.15.2 is able to successfully open the world.

Stack trace

20w09a\: [^crash-2020-03-04_10.51.58-server.txt]

Description: Exception in server tick loop

java.lang.NullPointerException: Exception in server tick loop
	at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:770)
	at com.google.common.collect.SingletonImmutableSet.<init>(SingletonImmutableSet.java:47)
	at com.google.common.collect.ImmutableSet.of(ImmutableSet.java:82)
	at bny.<init>(SourceFile:16)
	at bna.a(SourceFile:28)
	at cef.k(SourceFile:76)
	at xh.a(SourceFile:175)
	at blx.<init>(SourceFile:124)
	at xh.<init>(SourceFile:175)
	at net.minecraft.server.MinecraftServer.a(SourceFile:403)
	at egy.a(SourceFile:84)
	at egy.d(SourceFile:106)
	at net.minecraft.server.MinecraftServer.run(SourceFile:634)
	at java.base/java.lang.Thread.run(Thread.java:835)