Diff fragment saving was implemented as part of Review Board 3.0. It

allowed the contents of a loaded diff fragment container to be recorded

prior to loading new diffs from the server, in order to avoid having to

load data it already had. It would then be restored back into a

container.
The problem was that the container it was being restored to was expected

to be one already set up with the view for the diff fragment. If the

container was replaced, and that association lost, an assertion would be

hit during the load operation, breaking everything.
This change fixes this problem by enabling support for setting up a new

view when necessary. Instead of asserting the view exists, we assert

that the container exists (which is a requirement), and then we either

update and re-render the existing view or create a new one.
Unit tests were updated to handle this case for saved fragments.

Tested against a reproduceable failure case. No more assertions, and the

diff loaded.
Unit tests pass. Verified they failed with the same failure before the fix.