`comments-time-machine-links` errors on review page with pending reviews #2187

... and cause this incorrect dropdown:

<img width="803" alt="" src="https://user-images.githubusercontent.com/1402241/60288675-4e7c2f80-9947-11e9-9e4f-9d6f4ad35ff0.png">

❌ comments-time-machine-links
content.js:2596 TypeError: Cannot read property 'attributes' of null
    at comments_time_machine_links_init (content.js:5907)
    at details.init (content.js:2630)
    at features_run (content.js:2590)
    at content.js:2634
    at content.js:2542
    at onAjaxedPagesRaw (content.js:2537)

1. Start review
2. Reload page
3. Notice bug