Hi there Mike,
Thanks for the report. This is indeed a known bug we will be fixing in the next maintenance release of the plugin. We have a pull request that solves this, which you can use to patch the code in your site (let me know if you need further help on this).
You can also disable PHP notices by placing the line define( 'WP_DEBUG', false ); in your site’s wp-config.php file. More details on this can be found in the WP_DEBUG docs.
In any case I’ve added this thread to the bug report, so you’ll get a heads-up here when the fix is released 🙂
Best,
Nico