Put the summary of the bug you're having here

What I expected to happen was...:
Generate the terrain usually

What actually happened was...:
A never ending "Loading world" screen and a crash report being created.

Steps to Reproduce:
1. Click on "Create New World"
2. Select the world type "Superflat"
3. Customize using the "Water World" preset

4. Atempt to create this world