"cannot read property 'title' of null" when a PR description page contains resolved comments

Created by: lguychard

1. Visit a PR description page that contains resolved comments (eg https://github.com/sourcegraph/sourcegraph/pull/5712)
2. Witness error in console:

Uncaught TypeError: Cannot read property 'title' of null
    at getDeltaFileName (util.tsx:24)
    at Object.resolveDiffFileInfo [as resolveFileInfo] (file_info.ts:8)
    at MergeMapSubscriber.project (code_intelligence.tsx:551)
    at MergeMapSubscriber._tryNext (mergeMap.ts:141)
    at MergeMapSubscriber._next (mergeMap.ts:132)
    at MergeMapSubscriber.Subscriber.next (Subscriber.ts:100)
    at IntersectionObserver.<anonymous> (views.ts:135)

3. Click on the "files" tab (no hard reload)
4. Witness no code intelligence.