Hi,
Force quitting would normally be the remedy for terminating the background tasks and closing iMovie. When you reopen the project, the background tasks may start again since they weren't completed when you force quitted. The problem may be confined to just this one project.
Try opening and closing other projects normally and see if the problem exists there. If the issue repeats in other projects try deleting/resetting preferences and see if that solves it. Delete preferences even if the issue does not repeat in other projects on the outside chance that there is a preference issue with the one troublesome project.
To delete/reset preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new empty library. Reopen your old library to get back to your projects. (To reopen your old library close iMovie and go to your Movie folder or wherever you have your old iMovie library stored. Double click on the library’s icon. iMovie will open in that library.) Possibly you may need to
reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to
your project.
-- Rich