Hi, antoniof18,
Sometimes after updates preferences get disturbed and can cause issues like this. Try deleting 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 library. Reopen your old library to get back to your projects and see if you can import vids without crashing. Deleting preferences is a safe procedure that will not cause data loss or disruption to your projects.
If no luck, 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.
-- Rich