Same crash with HMI too.
I assume that you are running it on Windows?
Do you get exactly the same error (I'm curious about the stack trace specifically)?
And are you using only HMI or HMI and Hiver mods when it happens?
And

if Hiver is not enabled, can you try starting a new game without Nexerelin as well? I wonder if HMI is causing the issue or combination of HMI, Nexerelin and/or Hiver. If someone can confirm this for me there will be less code to check.
there should be a null check somewhere but dont ask me where it should go
That's the golden line

unfortunately I'm blocked exactly by this.
@volt woo hoo looks like we got some leads, [..] You got this!
Thanks for throwing me under the bus

but sersiously, thanks for the support, it always gives a push to try "one more thing".
At this point I decided to create the
issue on GH, if you have a GH account please comment there - it will help keep related info in one place,
If you don't have GH account (or some other reason) feel free to post it here.
I have some leads and ideas, but if I can't reproduce it on my machine it is really difficult to progress this, especially since I can't even tell if my fix is fixing anything ;p .
Unless someone with a Windows machine can have a crack on this, or provide more insignight on how Starsector works/where this error is coming from, we will need another idea on how to progress this.
PS One more to confirm, this happens on a new game, around the save, before showing the campaing screen with the fleet? And the game was started with: "New Game" > [generate name] 1 [continue] > 2 [skip] > 1 [proceed] > 1 [factions] > 1 [Hazard Mining] > [select any ship? I use large trade ;p ] > 1 [ensign] > 1 [done] > G [start game] > BOOM, game crashes?
PS2 @Dazs can you try testing version from GH -
https://github.com/jaghaimo/stelnet/issues/134 and paste logs, error stack trace and any lines containing "stelnet", you should see something like "ERROR stelnet.StelnetMod - GH134 onNewGameAfterEconomyLoad" and similar. I just noticed an errors that is related to your mod as well:
432484 [Thread-3] ERROR com.fs.starfarer.loading.StarfarerStrings - Missing string: [Nest asteroid field]
432484 [Thread-3] ERROR com.fs.starfarer.loading.StarfarerStrings - Missing string: [Isketot asteroid field]
PS3 @Dazs have you tried asking HMI maintainers about this issue? If they have any "unfinished" work that is possibly referenced? Because (not sure if related but) I see this in my logs:
359 [Thread-3] ERROR com.fs.starfarer.loading.WeaponSpreadsheetLoader - Weapon spec [radgun] not found in weapon_data.csv
520 [Thread-3] ERROR com.fs.starfarer.loading.ShipHullSpreadsheetLoader - Ship hull spec [junker] not found in ship_data.csv
520 [Thread-3] ERROR com.fs.starfarer.loading.ShipHullSpreadsheetLoader - Ship hull spec [flare] not found in ship_data.csv
520 [Thread-3] ERROR com.fs.starfarer.loading.ShipHullSpreadsheetLoader - Ship hull spec [module_hightech_decor] not found in ship_data.csv
520 [Thread-3] ERROR com.fs.starfarer.loading.ShipHullSpreadsheetLoader - Ship hull spec [hmi_auctor] not found in ship_data.csv
520 [Thread-3] ERROR com.fs.starfarer.loading.ShipHullSpreadsheetLoader - Ship hull spec [aurora_koe] not found in ship_data.csv
520 [Thread-3] ERROR com.fs.starfarer.loading.ShipHullSpreadsheetLoader - Ship hull spec [hmi_locomotive_front] not found in ship_data.csv
520 [Thread-3] ERROR com.fs.starfarer.loading.ShipHullSpreadsheetLoader - Ship hull spec [hmi_locomotive_base_lp] not found in ship_data.csv
If you want, you can try to fix those two errors from Hiver and add some arbitrary data to `ship_data.csv` for the ids above, and see if that help you, as I have no way telling the difference.