When trying to load a save that did not have SI previously enabled, I get this error:

Error while applying migration: Scrappy Industry: 0.10.0.lua

__scrap-industry__/migrations/0.10.0.lua:3: attempt to index field 'engine-from-scrap' (a nil value)
stack traceback:
    __scrap-industry__/migrations/0.10.0.lua:3: in main chunk

Tested with all other mods disabled and still happens.

Ah, hi starblaster! I must have wrote the migration wrong, I'll fix that in the morning

That migration is pretty old. I think the problem is that we renamed the recipe "engine-unit-from-scrap" but the migration's code still says "engine-from-scrap," so it's trying to reference a recipe that no longer exists. Most people (including us!) never noticed because the migration has either already been run or won't ever be run. You only see it if (as in this case) you're trying to add SI to an existing save.