I'm talking about this: https://github.com/Helfima/helmod/blob/f7033d053bee853025802bc6ad7cd6f88a318cc5/model/EntityPrototype.lua#L308
I think this is a very special case and maybe not really a "bug".
I stumbled upon Helmod printing "Entity is not boiler or fusion-reactor" to the game console, which I considered to be a very strange thing, especially since I couldn't find the source of this error string.
This happend when selecting the "steam assembling machine" introduced by Lignumis as a factory in a recipe in Helmod, I reported it there https://mods.factorio.com/mod/lignumis/discussion/67f684b219d0dfab4fe7b1e9 before I decided to dig through Helmod's code (just got curious what is happening).
Now, when tracing the path through Helmod's code until it breaks in EntityPrototype, I found out that maybe Lignumis' steam assembler is not correctly configured. I cannot claim with certainty that this is the case since I basically don't understand the first thing about modding Factorio, but the assembler seems pretty special, since they use a fluid as energy source (https://git.cacklingfiend.info/cacklingfiend/lignumis/src/commit/45f245d422ec3e2ba2d381c7f2840d195820943a/lignumis/prototypes/content/steam-assembling-machine.lua#L31), something which I don't think is used for an entity in vanilla Factorio and also not in all the other mods I use.
Maybe their assembler is (incorrectly) set up in such a way that Helmod (or Factorio) thinks it's a boiler or fusion-reactor and Helmod reaches EntityPrototype.lua#L308 correctly, or Helmod handles the case of an assembler using a fluid energy source not correctly. Looking at all the flags in the modding API you have to set to make it work together correctly, I tend to believe the former case.
However, in EntityPrototype.lua#L308, you have this:
self.lua_prototype.target_temperature or 0
The "or 0" part doesn't guard against the error, since I believe Factorio implements the restriction "Can only be used if this is Boiler or FusionReactor" in https://lua-api.factorio.com/latest/classes/LuaEntityPrototype.html#target_temperature such that they throw an error with the string "Entity is not boiler or fusion-reactor" (I have no idea where else should that come from), and the "or 0" part is never evaluated.
Maybe you could check whether the entity is a boiler or a fusion-reactor before accessing target_temperature - or you say that if Helmod reaches EntityPrototype.lua#L308 and the entity is neither of the two, the entity is set up wrong and thus the behaviour of Helmod is undefined and throwing the error is ok.
This is at least what I think I figured out - I could be totally wrong and there could be effects in play I didn't even consider.
