Hi there, thanks for this nice theme.
I have the latest Moodle version on 2.5.x branch (2.5.4+ Build: 20140207) with the Essential theme 2.6 (Build: 20131008) - the latest for Moodle 2.5.x.
If I log in or enter the home site as an administrator I get this error. If I reload then the site in my browser (checked in 4 different browsers), the error is gone in two of three cases until I enter the home site again. I know this is a known problem and it was fixed in higher theme versions > 2.6. But we can't change to Moodle 2.6 at this moment so we can't use theme versions > 2.6.
I searched this forum, but I didn't found a solution for this problem in Essential 2.6. How can I fix this error?