Can I ask, do you use a game pad or controller to play the game? I do and I have this exact same issue!
I was playing the "You have one day challenge" just so I could use the cheat/item spawner and go nuts testing out the guns and munitions, like the moltovs.
I waited for the hordes and standing on one of the rooftops, I hurled one over, only for the game to freeze, go black and crash. I thought it was maybe there were too many zombies when the same thing happened a second time.
I ran into a bug a while back involving the playable media not working for me and it turns out, it was an input bug linked to the controller. Maybe... this is another?