Version 0.9.9 is not compatible with the “Igrys” mod, so I had to roll back to version 0.9.8.

yeah 0.9.9 seems to break a number of mods (Paracelsin, Muluna, Igrys, Tenebris Prime), because you switched promethium-science-pack to research_trigger instead of science pack based (you require change_game_data.move_promethium_tech in data.lua line 15, which in turn redefines promethium-science-pack).

Those mods add additional science packs to unlock promethium-science-pack and try to access https://lua-api.factorio.com/latest/prototypes/TechnologyPrototype.html#unit, which isn't set anymore (see e.g. https://github.com/AndreusAxolotl/Paracelsin/blob/984406576404aa9b5dddce117c6824ceac38fe6a/prototypes/technology.lua#L582, crash because unit is nil).

Now, all those mods could check whether promethium-science-pack is still not research_triggererd, but as I have four mods that break because of the change in Nexis 0.9.9 I wonder whether you go too far in redefining the science pack in such a significant matter. Or, you can of course mark those other mods as incompatible.

Nevertheless, I'll report that in Muluna, too (I don't want to spam the other mods with bug reports, too, as of now), since they're pretty active and maybe have some idea how to cope with that...

Obviously, for now, I'll disable Nexus...

Edit: see here https://github.com/nicholasgower/planet-muluna/issues/317

I've reported it to the others as well as, as per the link to the official docs that stef77 put of https://lua-api.factorio.com/latest/prototypes/TechnologyPrototype.html#unit it is allowed to be nil and thus should be nil checked before access, which those mods are not doing thus violating factorio's API.

However, a fix on this mods side would be to leave the unit = {ingredients={}} field (and remove it in the data fixes stage, or potentially move anything that did get added to it to a higher up dependency of that tech), or to add an optional requirement on those 4 other mods (tenebris prime, paracelsin, igrys, and muluna) so you load after them and thus can freely and blindly wipe their changes anyway (but wouldn't work with potential other mods in the future as the first fix could do). :-)

References:

Muluna: https://github.com/nicholasgower/planet-muluna/issues/317
Igrys: https://mods.factorio.com/mod/Igrys/discussion/68ec19b633f50bf0a05cdcdf
Paracelsin: https://mods.factorio.com/mod/Paracelsin/discussion/68ec1ad133f50bf0a05cdce8
Tenebris: https://github.com/nicholasgower/tenebris-prime/issues/23

Thanks a lot for reporting!

Yep Confirming that this mod is the one breaking a whole bunch of other mods, I've downgraded to fix the problem

I will reset the technology to its previous state and rename my research from “promethium-science-pack” to “promethium-882-science-pack” to avoid further problems.

I have now only moved the “research-productivity.” I no longer get the error with the mentioned mod. If any problems arise, please report them.

Thank you very much for the report.