JS TypeError when unmuting on the BugTask:+index page

To reproduce:

 1. Go to https://bugs.qastaging.launchpad.net/launchpad/+bug/240067
 2. Click "Mute bug mail"
 3. Click Subscribe
 4. Select Unmute bug mail (the default) and click OK
 5. Notice that the Unmute link hasn't updated.

The error recorded is:

Uncaught TypeError: Cannot read property '_node' of null
    resolveNodeListFrom
    Anim
    Y.lazr.anim.flash_in
    Y.lazr.anim.green_flash
    config.on.success
    module.wrap_resource_on_success
    Launchpad.named_post.on.success
    b.Subscriber._notify
    b.Subscriber.notify
    b.CustomEvent._notify
    b.CustomEvent._procSubs
    b.CustomEvent.fireSimple
    b.CustomEvent.fire
    t
    B
    R

Which seems to be something to do with the green flash anim.