Viewer.js's inline destroy() only unbinds and leaves an in-flight image
view running; when it finishes it reads the destroyed item list and
throws "c is undefined". Closing the preview while an image was still
loading hit this in Firefox about once in 15 runs of the recorded-run
history test. A closed viewer is now destroyed once that view settles,
its image fails, or a 10 second bound passes.
The history test passed 60 of 60 runs in Firefox with the fix.
Vito Sansevero
5f21894196