When imported an exported Space, I can get excess (bad) external roots

(Bug, Closed -> Can't Repro, Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Import and Export
Summary:
Export the Spirit Island FAQ. Re-import it. Look at _currentSpace -> _externalRoots. You see the expected Simple Thing (and possibly others) from the original, but also Stylesheet (why?) and a bad link.
This has one nasty side-effect: it is causing Show All Things to fail with an error, because of the bad link.