Crashing can be caused by a number of things. Nothing to do but try some diagnostics to isolate the problem. Here's a link with some procedures that you can try:
https://support.apple.com/en-us/HT206157
Sometimes a crashing problem can be caused by something as simple as a corrupt preference. Try opening iMovie while holding down the Option and Command keys and selecting 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.
Also, there could be a conflict somewhere that is causing the crashing. You could 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