When placing a locomotive (modded ones included) on Muluna ( https://mods.factorio.com/mod/planet-muluna ) the game crashes from a non-recoverable error:

Error while running event PlantLib::script_raised_built (ID 94)
PlanetsLib/scripts/entity-replacement.lua:427: attempt to index local 'new_entity' (a nil value)
stack traceback:
PlanetsLib/scripts/entity-replacement.lua:427: in function 'replace_entity'
PlanetsLib/scripts/entity-replacement.lua:526: in function 'on_built_entity'
PlanetsLib/control.lua:184: in function <PlanetsLib/control.lua:179>
stack traceback:
[C]: in function 'create_entity'
more-quality-scaling/control.lua:321: in function <more-quality-scaling/control.lua:275>

This seems like an error with PlanetsLib, especially if it only happens on Muluna.

It only happens on Muluna, since I think it's something to do with how Muluna alters combustion based trains. (Space Trains (Maglev) do not cause this crash.) Likely due to both mods modifying the recently placed train simotaneously. If my memory serves, this only happens with quality trains. (not common)

I figured as much, but since the error occurs within the PlanetsLib code, this should probably be fixed on their end, rather than me disabling my mods functionality for trains on Muluna (which is the only thing I could realistically do on my side).

I've responded to this issue on Github. https://github.com/danielmartin0/PlanetsLib/issues/116

In short, I think the simplest solution would be for this mod to replace changes to acceleration and max speed in locomotives to use the quality features natively available in the Factorio engine.

1.7.3 fixes the crash, but now that I'm at it, I'll probably revamp the whole entity replacement system using the apply_upgrade method. Might take me a while though to integrate all the experimental branches I have currently lying around, so I didn't want to wait any longer with the crash fix.