You can't play customized worlds in 1.13

If you have a customized world from 1.12 you can't play it. I think you should add a locked customized world option so that players will be able to play their old customized world, but they won't be able to make a new one.

And now in czech for my translation issues.

Pokud máte customized svět z 1.12 tak ho nemůžete hrát. Myslím si, že by jste měli přidat uzamčenou možnost customized světa tak, že by hráči mohli hrát jejich staré customized světy, ale nemohli by udělat nové.