When my warrior clicks on a monster and brings up it's menu and then said monster leaves the screen, sometimes the box doesn't always disappear. If you interact with the box (such as inspecting) it fatal errors out due to trying to reference something that isn't there.
I usually handle when that happens. I thought it was fixed with the last update, but I haven't tried it since. That was exactly how I was getting a handle error and a crash. If I clicked the monster like that, it walked off screen, then I'd get the handle and the Realm would crash me out.