iMovie keeps on crashing...

Try creating a new user account and opening iMovie in the new account. That's to rule out that there is a conflict in your present user account that is causing the problem.

You can also try opening iMovie in a new library. Hold down the Option key while opening iMovie and a box will appear where you can select a different library or create a new library.

-- Rich

Redownloading iMovie would be one thing to try. Sometimes that cures issues. Before doing it there are a few other things you can try.

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.

Check to make sure that you have sufficient app free space on your drive for iMovie to function. Sometimes space can be used up without us being aware of it. Insufficient space can cause iMovie to crash.

Also, try the following procedures, as they are very easy to do and sometimes can cure crashing problems. It simply a matter of shutting down and rebooting while holding down certain keys.

Resetting the System Management Controller

Try resetting the System Management Controller (but no longer available for the new M1 computers):

https://support.apple.com/en-us/HT201295

Resetting the NVRAM or PRAM

Try resetting the NVRAM or PRAM:

https://support.apple.com/en-us/HT204063

-- Rich

If restarting the Mac doesn't cure it, try deleting preferences. Sometimes updates will cause preferences to become changed or corrupted. Deleting preferences will reset your preferences to the default ones. To delete 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.

-- Rich

Restart your Mac