Just updated a pack of mods and get this now when launching:
The mod Transport Drones (1.0.1) caused a non-recoverable error.
Please report this error to the mod author.
Error while running event Transport_Drones::on_load()
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function '__index'
__Transport_Drones__/script/depot_common.lua:324: in function 'get_lib'
__Transport_Drones__/script/depot_common.lua:329: in function 'load_depot'
__Transport_Drones__/script/depot_common.lua:482: in function 'on_load'
__core__/lualib/event_handler.lua:79: in function <__core__/lualib/event_handler.lua:74>
Lot of mods, but still fairly early and transport drones haven't been built as of yet (I "think" it's been researched though).
Disabling this mod, then loading the old savegame and migrating it then saving it out, then enabling this mod again then loading that save fixed it however, so it is fixed for me, but reporting this regardless.
From a quick check in the script, I'm not seeing how any depot's should be in the list at all since none have ever been built, so unsure what it is iterating over...