Adding a Bitbucket Server account to SourceTree 2.5 in MacOS is not possible

Summary

When trying to add a Bitbucket Server account to Sourcetree 2.5 (104), the Save button gets disabled. This happens for both SSH and HTTPS.

Steps to reproduce

1. In SourceTree, click on Cog icon > Add accounts > Add
2. In the 'Host' section select 'Bitbucket Server' and enter your URL. Note that the Save button is active
3. Enter your username. The Save button becomes disabled. No matter what you do now, the button will stay disabled and you won't be able to add your account

Workaround

If you fill in all the fields (except the username) and leave the username for the end, the Save button will remain active and you'll be able to add your account.