This has been happening to me for the past week. So frustrating. I have completed all missions and can now focus on adding detail to my Springfield. I have been adding detail as I go, and have rarely had crashing issues until very recently. Now the game is practically unplayable. I crash when assigning Homer, Lisa, and most recently, Barney. The game even crashes when I try to scroll through the decorations menu, so I can't even finish adding hedges. I don't even have all of the land nor any premium buildings (save for the sign and lair). I'm not sure how I'll be able to add more to my town in the future if I'm having this problem now. It took me two days to clear a modest amount of debris because of crashes.
So frustrating.