When you delete a folder or collection from the catalog, LR doesn't remove references to the folder or collection from the favorites of the film strip's source menu. It's impossible to delete the dangling references to collections from the favorites.
1. Make a new catalog and import one photo into it.
2. Make a collection containing the photo.
3. Select the folder containing the photo, click the film strip's source menu in the top left of the strip, and do Add To Favorites. Select the collection and do Add To Favorites.
4. Delete the collection and remove the folder from the catalog.
5. The source menu still shows the folder as a favorite. The menu now shows "Collection: Collection" as a favorite, rather than the original name of the collection.
6. Restart LR.
7. The favorite for the folder is now gone, but "Collection: Collection" still appears, and there is no way to get rid of it.
Tested on: LR 9.4 / Mac OS 10.15.5 and Windows 10.