Upload World in Java Realms gives error message "Cannot invoke "ctt.c()" because "this.b" is null

Affects versions 1.20.4, 1.20.3.

Duplicate of MC-267465

Attempting to upload a world in Java Realms -> Configure Realm -> Reset world -> Upload World gives the following error:

Noted as duplicate as I've located another bug with the same thing, but the wrong version.