I encountered this crash when working on fixing another mod issue I was having. It's consistently recurring after a save reload. The entire Nauvis base is waterfilled to give us some time to get to space, which is I assume what's triggering underground attacks. I can provide the save file if it helps, it's fairly heavily modded, the largest of which are Space Exploration and related mods.

Error MainLoop.cpp:1284: Exception at tick 20401290: The mod Rampant, fixed (1.5.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event RampantFixed::on_nth_tick(30)
RampantFixed/libs/UndergroundAttack.lua:187: attempt to index global 'universe' (a nil value)
stack traceback:
RampantFixed/libs/UndergroundAttack.lua:187: in function 'digOut'
RampantFixed/libs/UndergroundAttack.lua:369: in function 'processUndergroundSquad'
RampantFixed/libs/UndergroundAttack.lua:383: in function 'processUndergroundSquads'
RampantFixed/control.lua:1477: in function <RampantFixed/control.lua:1475>

I don't think you need a save to fix it. I will put an additional check in this place.

Release will be tomorrow

Also, in the release, it will be possible to disable underground attacks in the map settings

Thanks so much!