When accessing "More World Options...", game lags out

What I Expected to Happen
I expected to create a Superflat world with a custom preset.

What Actually Happened
As soon as I clicked "More World Options...", all hell broke loose. I was unable to click any ingame buttons. I tried two times and ended up with the same result. The exit button and minimize and maximize buttons still work.

Please fix this ASAP. And tell me what was wrong with the code.