you need to use blank reference files in the core mod.

A race that is not loaded needs the player entity but blank out as much as possible and make not select-able.

UI_FED-Units.brushes Core Mod

TXT
onlyLoadOnDemand FALSE
brushCount 0

UI_FED-Units.brushes in the Add-on

TXT
onlyLoadOnDemand FALSE
brushCount 267
brush
    name "PICTURE_FED_METALEXTRACTOR"
    content "Simple"
    fileName "UI_FED-Window"
    pixelBox [ 2 , 333 , 167 , 75 ]
brush
etc... etc... 

Keep all their buttons and textures on separate sheets, and call a blank brush file for the core mod. PlayerPics and Themes the same, files in the add-ons call the proper textures, but the core mod with no race loaded calls empty themes and pics and brush files.

The files all need to be in the manifests of the core mod but can be counts 0 and not call models and textures, let the add-ons handle that.

To have a mod with races in their own stackable mod, you can quite easily put all their entity (player file included), mesh, texture, and particle files in their own mod. While this will yield plenty of "failed to find data file" errors, it will not result in a minidump and the game will load just fine; this is how SoGE is set up for reference. I have not tried including Window/UI elements in the stackable mods, I've left them all in the core module, so I cannot speak for those; odds are they are what is causing you issues.

A core module must include the manifests which reference everything, even entity files that are not included in the core.

But that's wrong, you do not need the player entity in a core module, even if the race in question has entries in the GSD. I know this for a fact from my own experience.

PlayerKlingon.entity

TXT
entityType "Player"
raceNameStringID "IDS_PLAYERRACENAME_KLI"
canColonize TRUE
isPsi FALSE
isSelectable FALSE
selectablePriorty 3
randomCapitalShipNamePrefix "IDS_CAPITALSHIPNAME_KLI"
randomStarBaseNamePrefix "IDS_STARBASENAME_KLI"
entities
planetModuleInfo
Page:0
count 0
Page:1
count 0
titanInfo
Page:0
count 0
capitalShipInfo
Page:0
count 0
frigateInfo
Page:0
count 0
Page:1
count 0
NotOnPage
count 0
researchInfo
count 0

etc...

Load up Sins with all the stackable mods. Every. Single. One. If Sins doesn't dump, then it means you included something in one of your stackable mods that the core module needs. If it still dumps, something else is wrong. Have had this happen to me personally.

Awesome, glad to see you're on the right track now! While this is likely what you're doing, I'd keep loading the game with one less stackable mod each time, to see when Sins crashes. When Sins does crash, you've found one of the mods which has a file the core module wants.

So I was able to get the mods loading back up again (and updating to 1.51 in the process!).  I decided to try and use the dummy entity file thing to get rid of the dev.exe errors when I don't have all the race mods loaded but they are still popping up!

I have the entity player files in the core mod, with 0 ships and 0 reearches and are NOT selectable....However, the abilities/buff files, structures and techs still attempt to load and throw the "failed to find data file" errors.

Would someone be able to give me a little more detail on how to set up these dummy files?  Perhaps give me an example or something (a little more detailed the one already posted here)

Thanks!

^

I find the popups annoying regardless. Turn them off and turn logging on. It's the same net effect and scanning a log is more intuitive IMO.