Hey CJ, it looks like the error is coming from the Divi plugin or one of its extensions.
Indeed, if you google ET_GB_Block_Layout you’ll see a number of other sites have the same issue (In many cases present in the HTML being scraped by google). You should check you’re running the latest version of their plugin, and perhaps report this to Elegant Themes.
The reason the WP Discourse code appears (further down, which typically means it’s not the source of the issue) in the stack trace is because whenever you click the publish button any post-publication code will be executed, in an interconnected fashion.
If there’s any exception post-publication, whatever the source, the wp discourse plugin code is likely to appear at some point in the stack trace. It doesn’t mean it’s the source of the issue though.