How do you add subs to the game? (Like was done with the Tench and Narwhal in RSRD)

[Stuffer]

I'm trying to figure out how to add submarines to the game using the same method that was done in RSRD for the Tench and Narwhal.

I'm trying to add in the Type XXI submarine as its own sub instead of replacing the Type XVIII with it. I thought I had it down, but I hit a roadblock. Whenever I click the U-boat Campaign button in the main menu, I now get a CTD. Obviously I did something wrong or I'm missing some changes somewhere.

If you can give me a hand, I'd appreciate it. Thanks.