Your app/src contains error(s). Check console, fix errors, then refresh electron.
If error(s) are present in you app/src this creates conflicts with ESLint on first run. In turn, an INVALID webpack build.js is produced which interrupts HtmlWebpackPlugin from creating index.html. Since webpack-dev-server doesn't have the index.html ready to serve, the server falls back to the file explorer.
Make sure to close and reopen the developer tools panel on first launch if they are missing.