I'm experiencing a reproducible, consistent crash whenever I try to place a Loader in the editor. Doesn't seem to matter where I place it or which kind of loader. The actual crash is:

The mod Loaders modernized (2.1.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event loaders-modernized::on_built_entity (ID 6)
__loaders-modernized__/scripts/loaders-modernized.lua:49: attempt to index field 'fast_replace_variant' (a nil value)
stack traceback:
    __loaders-modernized__/scripts/loaders-modernized.lua:49: in function 'handler'
    __core__/lualib/event_handler.lua:72: in function <__core__/lualib/event_handler.lua:70>

Having a brief look, it seems to be happening at "local surface_data = storage.fast_replace_variant[entity.surface.index]". For whatever reason, it appears that the "storage" field is not being populated in the editor. I'm not up to tracking where the population takes place and why it might not happen, but that's the only explanation I can come up with. The loaders work perfectly fine in the actual game.

Some how that means your save got into a state that doesn't have the storage value appropriately initialized. The population takes place during loading a save after adding the mod, or starting a new save. It should be persistent. How are you getting into editor mode? Do you have any other mods loaded? Do you have the issue is you place the loader when NOT in editor mode? Is there an upgrade sequence in here somewhere - i.e. update Loaders Modernized from an older version - if so what version was it? Can you provide the save some where that is reproducing it?

OK, this is interesting. I have a "Free Play" scenario set up with nothing on it, used for testing mods. I don't remember when I made, but THAT is what's broken. Scenarios I create now don't have this issue, regular saves don't have this issue. It's just that one old scenario. To answer your questions:

• I get into the scenario from the main menu -> Scenarios -> Edit Scenario.
• I do have a fair few other mods loaded, but I don't think any of them are the cause.
• I do not have this issue in regular play, and I don't have it in new scenarios I create now.
• I don't remember what updates I've made to Loaders Modernized, but the only mod file I have for it is "loaders-modernized_2.1.3.zip".
• The scenario in question can be found here, just in case.

To be perfectly honest - if the issue is the one scenario I created, then I can just recreate it. There's nothing in the scenario that actually matters - it's a blank slate to drop buildings into so I can see if their pipe connections are in the right place and such.

This is definitely unique to scenarios. The startup cycle is not the same as loading a full game, so some of the migrations to happen. This is why the 'storage' data is not in a consistent state. In this case it will be easier and quicker to create a new scenario based on a current version of the mods you're using. I likely have to rewrite how I'm handling migrations soon anyway (the library I was depending on deprecated the method), but that will be a little while out.

Oh, yeah - I just made a new scenario and have been using that since. Honestly, I should probably just create a new scenario every time I need it, but going through new game settings is a pain. Either way, thanks for having a look :)