Customize Superflat crashes game

I was trying to customise the creation of a superflat world when the game crashed.

What I expected to happen was...:
I expected the game to take me to the 'Custom Superflat World' page.

What actually happened was...:
The game crashed

Steps to Reproduce:
1. Open Minecraft PC in snapshot 17w15a
2. Create New World
3. More World Options...
4. World Type: Superflat
5. Click 'Customize'