For the everyday Mac user, Apple has released an update to iMovie which fixes a bug with openeing files without a size affiliated with them.
"This Software Update fixes an issue with projects having a size of 0 KB. Attempting to open these projects would cause iMovie to quit unexpectedly at launch.This Software Update also addresses a problem where full-screen mode could not be accessed on some systems."
You can get the update via software update on your Mac right now.
