Crash when opening a world from 18w21b

In 18w22a, I tried to open a world created in previous snapshots and last tested in 18w21b, after displaying "preparing spawn", the screen went grey, then the game displayed "dsconnected" and crashed. The crash report is attached to the ticket.

New worlds created in 18w22a and worlds last played in 1.12.2 load without crashing, but it might still be an issue that needs investigation.