DownloadDiffAction inspects the current resolved URL in order to

determine its state. This assumes that a resolver match was made, which

isn't the case when displaying an error page, such as a 404.
The code has been rewritten to first pull out request as a

HttpRequest, which exposes the typing problem around the resolver

match. It then checks the match before assuming it's not None.

Visited a 404 page. Saw that I no longer hit the crash.