Conflict with WooThemes Storefront theme? Undefined PHP errors

I have used this plugin a couple of times and love it. However when I try to install it on any site running WooThemes’ Storefront theme, it throws-up errors both in the back-end admin and on the actual site.

These errors are:

Notice: Use of undefined constant RESS_POST_SHARING_UPDATE_COUNTS - assumed 'RESS_POST_SHARING_UPDATE_COUNTS' in /home/.../public_html/wp-content/plugins/responsive-social-share/inc/modules/post-sharing/init.php on line 14

Notice: Undefined index: _wpspec_cache in /home/.../public_html/wp-content/plugins/responsive-social-share/inc/classes/wpspec_Caching.php on line 76

Notice: Undefined index: _wpspec_lock in /home/.../public_html/wp-content/plugins/responsive-social-share/inc/classes/wpspec_Caching.php on line 77

All the sites I manage are quite similar, and the only consistent thing with this is it only happens on sites running with the Storefront theme.

Any help on what is causing this?

https://wordpress.org/plugins/responsive-social-share/