"Close source branch" is deleting branches even when the branch isn't merged causing data loss

When performing a merge operation online, we're able to select the option to "close source branch", which it's fine and desirable if the merge is successful.

But if the merge is not successful because conflicts need to be addressed manually, bitbucket is still closing the source branch, and it really really shouldn't.

When attempting (and failing to merge), that branch disappears, and if it was also deleted locally, we're unable to recover the lost work and resolve the conflicts manually.

This is actually very very dangerous because when the original submitter of the pull request publishes its feature-branch to the server, he assumes that since it's published, there's no need to keep a local copy of such branch and sometimes delete it. This causes situations where the work done is totally lost.

Maybe the application should even detect/predict that the merge will NOT be successful and not provide the option to merge at all, such as what happens on a pull request, where the "merge" button is disabled and unclickable in such situations?