A crash is most likely going to happen right after this report!
It seems bosses_of_mass_destruction:lich_tower | bosses_of_mass_destruction:lich_tower is the cause because it is not added
to the FlatChunkGeneratorConfig.STRUCTURE_FEATURE map. Please let the mod owner
of that structure know about this crash. That way they can add their structure
to that map since someone is trying to spawn it in a flat/custom dimension.

this was in the log

Thanks for reporting, I will investigate further

This should be fixed in version 1.2.2 - any existing superflat worlds won't be fixed. You will have to manually delete the structures from the level.dat if you really need it.