Cannot switch branches after pull, push, commit or merge

The problem arises EVERY time I try to change branches after a commit, a push or a pull. In order to change branches SourceTree has to be closed and reopened.

This is the dialog and error message that appears when trying to change branches.

The error points to an index.lock file in the .git folder, but upon checking there is no index.lock file. There is the index file, of course, which makes me wonder if SourceTree is not checking the filename for the .lock extension?

This error occurs in all of my repositories. Git used outside of SourceTree does not present any issues. The error started after updating SourceTree to v1.9.5.0, so I'm convinced this is a SourceTree bug.