I am getting familiarized with the Sideview Util editor. Recently I deleted the Appbar and some other modules and since then, I am getting this message as soon as I open the Sideview Utils editor. The two HTML modules aren't getting recognized while deleting as well as editing.
What this means, is that you deleted the "SideviewUtils" module from your view. This cripples all Sideview modules as well as the Editor.
Unfortunately the Editor lets you do it, and unfortunately, the error message you end up getting is a confused core-splunk error message rather than a more useful Sideview error message.
You'll have to manually put the SideviewUtils module back into your view (and remember not to remove it again).
Open the file on disk and add this
<module name="SideviewUtils" layoutPanel="appHeader" />