Delete Empty Folder in iMovie Library

iMovie 10: In the left margin, there is a folder called "iMovie Library". This folder has many subfolders whose names are years (2015, 2014, 2013, 2012, etc.). However, we have an empty subfolder called "3" at the very bottom of the list. iMovie gives us no way to delete this empty subfolder within the iMovie Library folder list. Any ideas? Please see screenshot below.