Unable to create '.git/index.lock': File exists.

I get this message very often when using sourcetree:

An unexpected error has occurred: CalledProcessError: Command: ('/Applications/Sourcetree.app/Contents/Resources/git_local/libexec/git-core/git', 'write-tree')

Return code: 128

Expected return code: 0

Output: (none)

Errors:

    fatal: Unable to create '/Users/sam/project/.git/index.lock': File exists.

    

    Another git process seems to be running in this repository, e.g.

    an editor opened by 'git commit'. Please make sure all processes

    are terminated then try again. If it still fails, a git process

    may have crashed in this repository earlier:

    remove the file manually to continue.

Usually trying again once or twice, it will work.

This looks to also be present on the Windows version of sourcetree, see https://jira.atlassian.com/browse/SRCTREEWIN-5639

I wonder therefore if this is some architecture problem with sourcetree?

This is on latest Sourcetree, 2.7.6