Hello James,
Thanks providing the url. I have checked the site now and it turns out that you have updated Cornerstone to the latest while using a very old version of the X theme.

Could you please update to the latest versions?
Our latest versions are:
- X theme
7.2.3
- Cornerstone
4.2.3
- WordPress core
5.3.2
This latest release contains fixes for several issues so be sure to check out the changelog (http://theme.co/changelog/).
After doing the updates, always remember to clear all caches (if you are using WP Rocket, WP SuperCache or W3 Total Cache) when updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
Please let us know how it goes.