iMovie must complete other tasks before performing this operation

I have tried closing iMovie for several weeks now and it has always said 'iMovie must complete other tasks before performing this operation'. I am unable to restart or shut down my computer because this notice will come up again and prevent it from shutting down. Quitting or attempting to fully delete iMovie results in the same message. I deleted all my iMovie files and the system files for iMovie (like different effects and sounds) and still nothing.