Hey, figured out an easy fix to this problem.
All you need to do is have an event available that the project can pull clips from. So, first, you need to create an event.
Pesky, though, because you can only create an event when you have clicked on "iMovie Libraries" in the left hand bar. Give that button a right click and you will see "New Event" a now viable option. Once you create an event, you should be able to add a project to that event.
Hope this helps! I didn't see anyone with this answer on the internet.