There is a conflict somewhere. You can either try some diagnostic things to isolate it or take your Mac into the genius bar at your local Apple store and let them worry about it. Your Mac should still be under warranty.
Here are a few easy things that you can try on your own, that often are done to root out conflicts:
Deleting Preferences
Try opening iMovie while holding down the
Option and Command keys and selecting to delete preferences in the box that
appears. iMovie will open in a new library. Reopen your old library to get back
to your projects.
Open in new Library
Open iMovie while holding down the Option key and select to create a new library from the dialogue box that appears. Create a new project in the library and see if you can work with it without the freezing up. If your library is stored on an external drive, the drive should be formatted Mac OS Extended (Journaled) or APFS, or your library may not function correctly.
Safe Mode
You could also try booting up in Safe
Mode (see Finder menu for instructions) and, after everything loads,
immediately shut down and reboot in normal mode. Then see if iMovie works
properly.
Resetting the System Management Controller
Try resetting the System Management
Controller:
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM,
although this issue would be fairly rare:
https://support.apple.com/en-us/HT204063
Redownloading iMovie
If none of the above work, try redownloading iMovie.
To update/redownload iMovie 10, drag your
iMovie 10 app from the Applications folder into the trash, but do not empty the
trash. Sign in to the app store and open your purchases folder by
clicking on your name at the bottom of the app store side bar. You will see
your purchases displayed, including iMovie 10. Redownload/update iMovie from
there. If not successful, drag your present iMovie app out of the trash and return it
to the Applications folder.
If still no luck, then the genius bar would be your next stop.
-- Rich