Hello,
I’m getting this error when trying to access the theme customizer. All plugins are disabled except for Cornerstone. When Cornerstone is activated, this error shows up again. So I’m very sure this is a Cornerstone problem. The site is new and hosted on Flywheel, so it’s all up to date and using the latest version of X Theme.
Is this a known bug or is there a release that will fix it soon? Also, is there additional information needed from me? I can deactivate and reactivate Cornerstone when editing things in the Customizer for now, but this will be handed off to a client and that manual switching can’t be a permanent solution.