Summary

When merging a pull request fails, the UI does raises and error and the loading spinner never goes away:

#!log

Uncaught TypeError: Cannot read property 'responseJSON' of undefined
    at f.error (merge-button-dialog.js:48)
    at Object.<anonymous> (merge-button-dialog.js:43)

Please note, fast forward merges are only possibly if no changes have been made to the destination branch.

How to Replicate

1. Open a PR in a browser window
2. Outside the browser window, update the destination branch to introduce a merge conflict
3. In the open window, try to merge the PR
4. The loading spinner will remain indefinitely