With crashing issues, nothing to do but try a few things to see if you can isolate the conflict.
Try shutting down your Mac and rebooting in Safe Mode. (See the Finder help menu for directions.) Once everything loads, immediately shut down again and reboot in normal mode. See if that cures the problem.
Possibly there is some corruption in the movie that is causing the crash. Or the issue is confined to one project. Try creating a new project and populate it with different media from the first project and see if you can successfully export.
Check to see that you have sufficient internal drive space for iMovie to operate efficiently, and sufficient space on the destination drive to receive the movie.
You can also try creating a new library, copying your project to it, and seeing if it will share out from there.
Finally, if still having crashing issues, 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.
All else failing, try contacting Apple Support. The techs there have the capability of remotely accessing your computer that we cannot do here. To contact apple support click on the Contact Support item in the upper right hand corner of this forum’s screen. Navigate to a place where you can enter your phone number.
-- Rich