Generation of new chunks fails with ""☃" is null" on imported world

Put the summary of the bug you're having here

What I expected to happen was...:
Go to the "border of my map" to see the transition between old and newly generated chunks. 

What actually happened was...:
Server throws expection and crashes

Steps to Reproduce:
1. Import an existing 1.16 map
2. Teleport close to the border of already generated chunks
3. Get closer to the border -> rendering fails, player gets stuck in void