if fluids[name] or not fluids[name].valid then
      insert{name = name, amount = amount}

This inserts fluid if given fluid exists... or when the fluid doesn't exist and is not valid. Unsurprisingly it crashes on second option.
You probably meant:
if fluids[name] and fluids[name].valid then

Also with debug in spawning.lua in spawn_entity():

  local e = surface.create_entity
  {
    ...
  }
  if debug_log then log(string.format("[spawn_entity]: Entity created: e.valid='%s'", e.valid)) end

The debug log crashes when e doesn't exist.
This happens in a ruin with 16 entities, for enemy artillery-wagon and enemy fluid-wagon. Probably 'Realistic Ruins' ruin set.

Also with debug in spawning.lua in spawn_entity():
local e = surface.create_entity { ... } if debug_log then log(string.format("[spawn_entity]: Entity created: e.valid='%s'", e.valid)) end
The debug log crashes when e doesn't exist.
This happens in a ruin with 16 entities, for enemy artillery-wagon and enemy fluid-wagon. Probably 'Realistic Ruins' ruin set.

Ah:
https://lua-api.factorio.com/latest/classes/LuaSurface.html#create_entity

Return values

The created entity or nil if the creation failed.

Okay, so e.valid will fail when e is nil.

And I don't see any proper handling of e being nil. It just proceeds and maybe invoke other functions where nil might be stopped.

And I fixed the above one. Now I just wonder why there are local copies of prototypes.fluid and entity.insert_fluid()?

I have removed the local copies (no need, just reference the original directly) with new release 1.3.9.