Crash after customizing superflat world with a custom preset

The bug

After clicking "Use Preset" with a custom preset with an invalid character, it crashes.

What I expect to happen

A message will appear showing that the preset is invalid.

Steps to reproduce

Try to create a new superflat world with the preset minecraft:piston[facing=up];1;

Note: This may be a duplicate of MC-122441, but it doesn't seem to be fixed.