The game breaks and freezes when you optimise a world from 1.12 to 1.13 with the original world being in the nether.
TO REPRODUCE:
Create a creative world in 1.12 and go to the nether.
Open the 1.13 menu and optimise the world.
Open the world in 1.13
Is this still an issue in the most recent versions (currently that is 1.13.2, or the latest snapshot 18w49a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.