When the diff revision sliders are moved before all the files on a page finish

loading, it can cause a javascript error. The cause here is that the

DiffReviewable has an outstanding fetch, which attempts to render the file once

it finishes. If the element disappears, we try to dereference things which are

now undefined. I've made it test to see whether the target element is present

before creating the DiffReviewableView, which avoids all of the problems.

Flipped around to a bunch of different diff revisions, changing things before

all files loaded. After this change, everything works swimmingly.