https://bugs.webkit.org/show_bug.cgi?id=162254 made URL.origin null if a scheme is not handled. That may make the Web Inspector look strange if resources are loaded from such a scheme.
<rdar://problem/71263166>
I now see that this probably is necessary. A WKURLSchemeHandler may be installed on the WKWebView being inspected (where the load would succeed) but not in the inspector's WKWebView (where the origin would be null).