"null.prompt" pop-up when creating branch from JIRA.

Summary:

When trying to create a branch through JIRA Development Panel, a pop-up with a 'null.prompt' will be showing instead of getting redirected to Bitbucket.

Steps to Reproduce:

1. Have JIRA integrated with Bitbucket Server.
2. Access JIRA using Firefox.
3. Navigate to any of JIRA Tickets.
4. In the Development Panel click "Create Branch"

Expected Behavior:

The user will be redirected to Bitbucket to create a branch.

Actual Behavior:

A 'null.prompt' pop-up will appear instead.

Notes:

• At the moment when I replicate the issue, so far it is only possible through Firefox using my Machine.
• Tested with JIRA 7.8.0 and Bitbucket Server 5.8.0
• With JAC is running on 7.8.0, the issue itself could be replicable here:
  
• From my observation, when the feature works, the issue would not happen anymore until I clear my browser cache.