"Almost done! Fiddling with results…" is not finishing

I have been running DupeGuru over a large set of folders. Over the process of several days it found over 1.3M duplicates. Now instead of displaying the results window and deleting duplicates, I the progress bar is stuck at the end with a message "Almost done! Fiddling with results…".

DupeGuru has been like this for hours now. There is no process indication. DupeGuru uses <1% CPU. It remains at ca. 1.2 GB RAM as has been the case for most of the duplicate search. There is also no disk activity. So it appears like DupeGuru does nothing.

I am running this on a Mac with macOS Mojave 10.14.1.