This has happened a couple of times to me too, however there doesn't seem to be any way to reliably reproduce it, other than it seems to be linked to opening full screen UI menus and dialogue boxes. Opening lots of menus in a short time span seems to increase the chance of it happening. The workaround I've found when it happens has been to do "Alt+F4" or press the x button on the top right corner if in windowed mode (not sure of the equivalent on Mac) and it should give you the option to "save and quit". Then before reloading the game, clear the cache files. Not ideal but it prevents you from losing progress when it happens.
Omg I LOVE YOU. You are amazing. Would have lost so much progress. Didn't mind restarting the game just wanted a way to save before I closed. God bless you.
