Crash when creating flat world with special characters in preset

Create a flat world, and in "presets" type any non-letter character and hit "done"

This also make impossible to have precises states in the blocks of the world created because if you want for example "_minecraft:oak_log[axis=x]_" it will crash because of the _[_

Steps to reproduce:

Make a new world, options "flat world", customize and add any character like _[\\%}_ at any position and hit "done"