There is a good working workaround, posted by “bastiencyr” on Bountysource.com
I have a provisional solution to empty the trashbin. Do this :
1. select the mark all button and suppress all (only files NOT in group folder will be removed)
2. select the mark all button again, and uncheck only one file on the list.
3. Remove all files (except the one last file)
4. it works for me