See screenshot attach.
Jan Doe created custom widget
Jan Doe created a field under ‘Content editing fields’ and set to ‘text’ (for $example)
Jan Doe saved and deployed
Jan Doe added contents in the webpage
Jan Doe decided to edit the custom widget by replacing to ‘rich text’ (and forgot to keep it consistent $example but instead called it $exampleFoo)
Error message appeared on the webpage.
To make it less confusing, instead of wordings “Error processing inline link Content is not allowed in prolog.” and maybe just say like “Please delete the content and re-enter the new content” or something like that. That’s how the error message disappeared.