While using SourceTree I renamed the two main local branches (master and develop).
However, when I now click on the Git Flow icon to start a new feature, it tells me that the repository needs to be initialized (even though it has been already been initialized).
REPOSITORY -> GIT FLOW -> INITIALIZE REPOSITORY
Shut Sourcetree, and edit the file .git/config updating the [gitflow "branch"] section. (the .git directory is sometimes hidden within file explorer)
The default will look something like this:
[gitflow "branch"] master = master develop = develop
And you should change it to match your new branch names, i.e.:
[gitflow "branch"] master = yourNewMasterBranchName develop = yourNewDevelopBranchName
Then restart SourceTree