Comment 1 for bug 101240

Is this even desired? Locally, I've altered kupu so it displays a message if ES isn't in the installed extensions list. I can do something similar for widgets, but that would be after the source element is already added (you can't conditionally hide a widget, so it's always available).

Consider this:
An import of a publication is done into a fresh silva without external sources installed. A document in the import contains an external source. Should the source in the document be able to be removed or edited (in kupu?) It'll always be removable in kupu. Verify that existing ES's _can_ be edited / removed if ES is not installed . Perhaps also display a note in kupu tool and widget informing editor that ES is not installed.