This does seem to happen after doing a pull/push.

After attempting to switch branch, and the error message showing, when I watch the .git folder the index.lock file is repeatedly added and removed and so the ability to then switch branch without first closing sourcetree depends on timing.

Before the error message is shown for the first time, there is no index.lock file.

The problem exists both when the "Use Embedded Git" and "Use System Git" options are selected. The embedded git version is 2.6.1.