Error displayed while adding a developer reply for second unlisted version of an add-on

[ValentinaPC]

Steps to reproduce:

1. Submit an listed i.e. add-on https://addons-dev.allizom.org/en-US/firefox/addon/source2/
2. Update the add-on with an unlisted second version
3. Go to Manage Status & Versions and give a Reply from Review history for the unlisted version

Expected results:
No error displayed.

Actual results:
A new tab page containing the message "{"detail":"Authentication credentials were not provided."}" is opened.

Notes/Issues:
Not reproducing if the add-on has only unlisted versions.
Verified on AMO-dev FF50 (Win 7)
Video for this issue:

#3481

[eviljeff]

Also part of #3607

[ValentinaPC]

Verified as fixed on AMO-dev FF50 (Win 7)
Postfix screenshot: