(I am indeed aware, this topic is a duplicate of many others, exept it is in a newer version.)
The example seen here is an event of trying to convert a 1.14 world to 1.14.2.
Environment
Happens when either creating a new world or trying to convert one from older versions.
Undependent on wether it is a flat or a normal world type.
Using Java 8 Update 45, as it's requested to downgrade when this issue happens.
Duplicate of MC-139271