Crash when clicking customize for superflat worlds.

When I create a new world, switch the type to superflat and click customize, the game turns to the dirt background for less than a second then crashes. Nothing is left in the launcher output log. However MultiMC spews a bunch of:

at byo.a(SourceFile:306)

To reproduce:

Start 17w15a
Create a new world, more world options, switch to Superflat, press customize.

Logs included.