Before doing anything more, read through this thread and see if anything in there helps you:
https://discussions.apple.com/community/ilife/imovie
Downgrading to a previous operating system can be tricky. Since I have never
done it myself I am reluctant to try to provide instructions on a forum like
this. I suggest that you click on the Contact Support item in the upper right
hand corner of this forum's screen, and talk to an Apple Support person who can
walk you through the process.
For information purposes you can read this Apple Support article re restoring a
previous operating system from a Time Machine backup.
https://support.apple.com/en-us/HT203981
Rather than trying to downgrade your operating system I suggest that you wait for whatever patch that Apple may do, and try some of the diagnostic procedures mentioned in the discussions link above.
-- Rich