Looks like it wasn't that :/
I started up a fresh game and I ended up getting the error again. It occurred about halfway through the primaries and at a different spot to boot.
I've already had a successful run through with this primary order :/ (also this error occurred near the end of the primaries). One thing I've thought of that may cause the problem: I changed the some of the stuff in the scenario folder in the middle of the game. It still went on for several turns/primaries after that, but could that have caused the crash?
I got an error that said DoMomentumEffects::EOverflow. It was in a scenario that I was creating. So far all I changed was the primary order, the start date, and the percentage needed in the Dem side to be eligible for delegates (from 15% to 5%).