Hello mid-century guy,
Welcome to the Apple Support Communities. I understand you've been running into some issues in iMovie.
Starting with saving your projects, iMovie should be saving your work as you go automatically, so there's not a save option like you'd find in some other apps. This is outlined here for reference: iMovie for Mac: Create a new movie. Are you able to re-open your projects from the Projects view in iMovie? If you're finding your projects aren't saving and iMovie shuts down while in use, there may be a few factors to look at.
First, what's the exact text of the error message you receive when iMovie shuts down? Does it only shut down when you trying to send via email, or under other circumstances as well? To look at it from another angle, are you able to send email as expected through the Mail app at the moment?
The best place to start is to make sure iMovie is updated. To do this, open the App Store and click Updates; if any appear for iMovie, install those, and then test it out.
If this doesn't resolve the issue, click Apple menu () > Restart and give it another try. Feel free to report back to the Apple Support Communities with the results.
Cheers!