I recently upgraded Thinderbird and now the Remove Duplicates add on wants me to specify a folder but doesn't seem to give any way to do so

I recently upgraded to 140.12.1. When I attempt to run the Remove Duplicate add on it hangs. I suspect this is something to do with an additional menu item that now lists under Remove Duplicates Saying "Set original message folder(s) for next duplicate search". However when I select this, I get an error message from Remove Duplicates saying "Invalid choice of original message folders." I have no idea where to set the folders, nor what to set them to..Can anyone help - it's an excellent add on which I use a lot.