Error: 'NoneType' object has no attribute 'sim_info': when this came up in my game, it was because one of my sims was culled between making the .json file and replacing the trait. I managed to locate the entry in the list that was an issue by searching for the last trait that was replaced (since sim-trait pairs are grouped by trait), finding the entry that has that trait and a sim ID that isn't in the game, and just deleting that entry.
It's kind of a bodge honestly, so if this issue keeps coming up for others I can try to write a bit more code to find these and remove them?