My goal is to add just train ramps to an existing save.
In mod settings, I unchecked everything except "Enable Train Ramps". (There's nothing on it or it's tooltip about requiring any of the other options to be enabled.)

When I load a save with this configuration (a save that's never had this mod before) I get this:

Error AppManagerStates.cpp:1591: Error while applying migration: Renai Transportation: RT-0.2.0.lua

RenaiTransportation/migrations/RT-0.2.0.lua:11: attempt to index field 'FastThrowerInserterRecipie' (a nil value)
stack traceback:
RenaiTransportation/migrations/RT-0.2.0.lua:11: in main chunk

If I then check "enable base throwers/hatchets/bounce pads" (in addition to the above option), I get a new error:
migrations/RT-0.2.1.lua:10: attempt to index field 'PrimerBouncePlateRecipie' (a nil value)

(As a temporary measure I just deleted the whole migrations folder to get it working. As I said, this save never had this mod before, so there's nothing to migrate.)

I see, settings to enable/disable weren't added for a while so all the migrations must be looking for things that should be there but aren't if disabled by settings. I'll go through the migration files and see what needs some valid checks

I made a new save without the mod, then tried loading it with the mod to recreate the crash and confirm it. Then after I added some extra checks to those two migration files it did not crash anymore, I looked through the other migration files and I dont believe there are any other problematic sections in any of those but if you run into any please let me know. It should be good to go for anyone else in the future