Caused by the fix of MC-132281.
The bug
You won't be able to modify / write in the writing box while customising a Superflat world's preset.
How to reproduce
Create New World
More World Options...
World type: Superflat
Click on the option "Customise"
Remove all the layers (optional)
Click on the option "Presets"
Try to modify / write in the writing box
Linked issues
is duplicated by 11
MC-132643
Can't edit superflat preset text
Resolved
MC-132697
Unable to type or delete anythinbg inside
Resolved
MC-132729
You can no longer type in the Superflat Preset box
Resolved
MC-132785
Can't customize preset
Resolved
MC-132929
Superflat Custom Preset Text Box Not Editable
Resolved
6 more links
Fry
Can confirm, I really want this fixed because now I have to use an NBT editor to edit it.