Possible to navigate to raw JSON when merging MR & removing source branch

I can reproduce locally and on GitLab.com with the following steps:

1. As an administrator, navigate to an MR you can accept
2. Select "Remove source branch" and accept the MR
3. Wait for merge to finish, then hit the browser's back button 1-2 times

I cannot reproduce the bug when "Remove source branch" is not selected.