I may be having the same issue. Can’t double-check until tomorrow, but I couldn’t close the editor after I imported an .fbx via right-clicking in the Content Browser. This is on 4.16.2. I imported several .fbx models yesterday in 4.16.1 without issue, but I updated to 4.16.2 this morning and that was the only time I tried importing a model since.
I couldn’t close through clicking the X, nor through selecting Quit from the File(?) drop-down menu. I also couldn’t open another Recent Project. Trying any of these actions just resulted in nothing happening, as far as I could tell. I had to End Task in Windows Task Manager. Haven’t had a chance to check again, but I will tomorrow.
EDIT: I can’t seem to replicate the issue today. I was able to import (and re-import) a new .fbx in 4.16.2, then I was able to save and then Quit without a problem. I did all that three times today, but maybe I’m just getting lucky, or there is some other specific thing I haven’t done today which causes the issue to present itself.