Version 2015021100 was release 2.8.1.1-RC3 which was never tagged, announced etc. And certainly should be no where near a production server. The error of which you speak was a fault I made during development and then later corrected. Unless testing, please only use tagged stable versions not pre-release from https://github.com/gjb2048/moodle-theme_essential/releases or https://moodle.org/plugins/view.php?plugin=theme_essential.
Gareth