To Reproduce:

1. At the command line, cd to an existing git repository.
2. Create a new orphan branch (a branch with no parent) by executing 'git checkout --orphan NAME_OF_NEW_BRANCH'.
3. Open this repository in SourceTree, e.g. by typing 'stree .'

Actual behaviour:
SourceTree frequently displays an error dialog with the message: "'git log' failed with code 128: FATAL: bad revision 'HEAD'." These error messages don't go away until you make an initial commit on this orphan branch.

Expected behaviour:
Nothing out of the ordinary.