Switching branches fails

when switching branches with double clicking on the node it fails with:

git -c diff.mnemonicprefix=false -c core.quotepath=false checkout feature/LIB-589_issuename
fatal: Unable to create 'C:/Source/qps-source/.git/index.lock': File exists.

If no other git process is currently running, this probably means a
git process crashed in this repository earlier. Make sure no other git
process is running and remove the file manually to continue.

around 50% of the time; however right click -> checkout always works

(sourcetree is the only app accessing the git repo at the time)

our theory is that its because the branches are far apart that updating the right panel takes to much time (as the context menu always appears after that panel has been updated)

its just annoying to us as source tree is our only git app