I've seen this issue as well. It seems to correlate with another glitch -- after I had created a new kingdom, my clan spontaneously left the kingdom for no apparent reason. I re-formed the kingdom with the same name, and it happened again a couple of days later. Finally I just left myself out of the kingdom for a while, and after a couple more days, I got a crash with the exact same stack trace.
I'm also using the Marry Anyone mod. I'll try reproducing it in a clean save with no mods, but I'll have to cheat a bit to bypass the lengthy grind to Clan Tier 4.