Hi there,
Developer of Simple Calendar (a Google Calendar plugin) here. See https://wordpress.org/plugins/google-calendar-events/
A mutual user of ours discovered that activating both our plugins will take down a site with a bunch of PHP errors. I confirmed in a test.
Taking a look at your code, we both use Composer autoloading to bring in PHP libraries.
However, we include the Google PHP API client, and it appears you do not. But you DO reference it.
Take a look in /lib/vendor/composer/autoload_classmap.php of the latest version of your plugin, or do a search for “/google/apiclient/” in your code.
If there’s something we should add to our code we’re more than happy to. Just submit a PR or open an issue at https://github.com/moonstonemedia/Simple-Calendar.
Thanks,
Phil