Bug reproduction steps:

1. Create a new repo (like all new repos, it will start out empty).
2. Do NOT add any content to the repo,
3. fork the empty repo.
4. Add content to the forked repo.
5. Now try to generate a pull request from the forked repo to the new (still empty) repo.
6. You will see a page that says "Something went wrong"

I consider this a bug for these reasons:

1. From a git standpoint this should work fine (and does on git hub or gitlab)
2. This is a common situation for our work flow (where a repo admin creates and controls master repos, but all content is generated by folks other than the admin).

Workaround:
Add any dummy file to the new repo before the 1st fork is made: then the pull request will process normally. However, this litters the repo with an uneeded file just to avoid the bug.

More info is in bitbucket support request BBS-6783
I opened this issue at the suggestion of the Atlassian support.

Dave