To reproduce:

• Create a branch locally, commit some changes
• Push to remote (Bitbucket) and create a PR
• Rename your branch locally and remote following this guide: https://multiplestates.wordpress.com/2015/02/05/rename-a-local-and-remote-branch-in-git/
• Try to update your PR by clicking edit

The page at bitbucket.org/{username}/{repository}/pull-requests/update/{pr} will be broken. Showing JavaScript errors in the console, an endless loading spinner where the code is usually displayed and a broken branch selector on the source branch selector.