This was reproduced with all Add-Ins. There are script conflicts with the latest Microsoft Updates of SharePoint online.
If you have a behavior that an Add-In (skybow Rich Forms, Tabbed Pages, List View Search, Theme Designer, Action Links) is endless loading and in the console of your developer tool are such errors,
then you need to update to the latest Full Trust version of your specific Add-In.
(Unchecked lastError value: Error: Could not establish connection. Receiving end does not exist, t is undefined)
List View Search
Rich Forms and Theme Designer have similar behaviours