The issue you're having is due to the incompatible version of X with Cornerstone as you're using an old version of X with the latest version of Cornerstone. Always make sure to use the compatible versions and update both X and Cornerstone to latest version to avoid such conflicts and potential errors. You can find the latest version numbers here: (http://theme.co/changelog/) Then you can compare them to what's installed on your site.
If you find anything to be out of date, you can review our update guide.
Updating the theme will fix the issue, don't forget to clear all cache (plugin, browser or CDN) before and after updating the theme.