Thank you so much! Glad to have a copy of Donsol & a better understanding in case I run into this again :)
The "package" and "package-lock" .json files had specified the newest versions of electron, so I changed those. When I installed electron-packager, it downloaded v7.3.3, which must be the most recent compatible version with the source code. I'm on Windows 10 in case this is helpful for anyone else :)