Error: editor not found

[lguychard]

1. Navigate to a PR, eg. https://github.com/sourcegraph/sourcegraph/pull/3548.
2. Switch between the Conversation and Files tabs.
3. Witness errors like this in the console:

Uncaught Error: editor not found: editor#0
    at findEditor (EditorTextField.tsx:202)
    at MapSubscriber.project (EditorTextField.tsx:68)
    at MapSubscriber._next (map.ts:85)
    at MapSubscriber.Subscriber.next (Subscriber.ts:100)
    at MapSubscriber._next (map.ts:90)
    at MapSubscriber.Subscriber.next (Subscriber.ts:100)
    at CombineLatestSubscriber.notifyNext (combineLatest.ts:280)
    at InnerSubscriber._next (InnerSubscriber.ts:18)
    at InnerSubscriber.Subscriber.next (Subscriber.ts:100)
    at BehaviorSubject.Subject.next (Subject.ts:71)

When calling removeEditor() when unsubscribing from synchronizeTextField(), editorService.editors emits, and the call to findEditor() in updateElementOnEditorOrModelChanges() throws.

cc @sqs