fix loading URL children

This is why `Current Web Page ⇥ Search Contents` wouldn't work. If you just ask for a URL's children without ever calling `objectHasChildren:` first, the handler would always be `nil`.

Generally, I think we should discourage making changes to an object when simply asking if it has children, but I can see why it was done in this case. Since we can keep the work-around local to the class that creates the "problem", I think it's fine.