Got this message;

The mod The Ruins Mod (1.0.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event AbandonedRuins::on_tick (ID 0)
The mod RealisticReactors (2.18.17) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event RealisticReactors::on_entity_died (ID 4)
RealisticReactors/scripts/construct_destruct.lua:107: attempt to index field '?' (a nil value)
stack traceback:
RealisticReactors/scripts/construct_destruct.lua:107: in function <RealisticReactors/scripts/construct_destruct.lua:41>
stack traceback:
AbandonedRuins/utilities.lua:40: in function 'safe_die'
AbandonedRuins/spawning.lua:51: in function 'spawn_entity'
AbandonedRuins/spawning.lua:68: in function 'spawn_entities'
AbandonedRuins/spawning.lua:131: in function 'spawn_ruin'
AbandonedRuins/spawning.lua:139: in function 'spawn_random_ruin'
AbandonedRuins/control.lua:44: in function <AbandonedRuins/control.lua:40>
stack traceback:
[C]: in function 'die'
AbandonedRuins/utilities.lua:40: in function 'safe_die'
AbandonedRuins/spawning.lua:51: in function 'spawn_entity'
AbandonedRuins/spawning.lua:68: in function 'spawn_entities'
AbandonedRuins/spawning.lua:131: in function 'spawn_ruin'
AbandonedRuins/spawning.lua:139: in function 'spawn_random_ruin'
AbandonedRuins/control.lua:44: in function <AbandonedRuins/control.lua:40>

Thanks for the lovely work!!

This looks like an incompatibility between the Ruins Mod and the Realistic Reactors mod. But the error is caused by the Ruins Mod, you need to ask the mod author about this...

They already asked me about this, I told them it looks like an error with your mod. It looks like an error with realistic reactors because I am just calling .die() on an entity and realistic reactors crashes in the event that is raised by that. Since the event is raised by the game, a crash in there can really only be caused by the mod listening to the event.