Hi, Krabsens,
Crashing can be caused by a number of different things. It could be a corrupt preference. Or maybe not enough disk space for the iMovie app to function properly, or perhaps not enough space in the destination drive to receive the movie. It could also be a corrupt clip in the project that is triggering the crash. Or it could be an issue confined to just one project. Or there might be a conflict in the particular user account that you are using. Possibly it could be an issue with your computer, such as bad RAM or maybe a failing graphics card. Or maybe your iMovie 10 app is not updated to the highest version that Sierra can run. No way to tell without doing some diagnostic procedures.
Try doing the procedures below to see if you can isolate the problem:
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. Deleting preferences is a safe procedure that
will not cause data loss or disruption to your project.
Opening iMovie in a new library
Try opening iMovie in a new library. To do
that, open iMovie while holding down the Option key and then selecting to
create a new library in the box that appears.
Opening iMovie in a new user account
Create a new user account from System Preferences/Users and Groups and try to open and
use iMovie in the new account. You can use the Guest account on your Mac for
this purpose.
New Project
Another thing you can try is to copy your media into a newly created project and see if
that cures the problem. To do that, do an Edit/Select All, Edit/Copy, of the
media in your project and then Edit/Paste it into the newly created project.
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.
Updating 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.
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
Apple Hardware Test
To rule out a hardware problem, try running the Apple Hardware test on your Mac.
https://www.idownloadblog.com/2016/03/07/how-to-start-mac-apple-diagnostics-hardware-test-mode/
-- Rich