When a process is complete in Acrobat (Optimize PDF), a dialog box pops up behind the main window

This dialog locks the program, and clicking anywhere in the window just gives a "ding" sound, and I can't close this phantom dialog box.

I end up having to open Task Manager and Ending the Acrobat DC program, losing all of my work.

How can I get to this box so I don't keep losing hours of work at a time?