Tortoise commit the git-lfs files, show me a error: ".git/index.lock" file exist.

with git lfs enabled, now I got a strage problem, here is the steps:

1. git clone https://gitlab.com/sancho87/lfs-test.git to my local directory named lfs-test.
2. create a new test.tar, and copy it to lfs-test to replace the older test.tar file.
3. right click, select the Git Commit->master menu, write some logs and click OK button.
4. sometimes, not usually, TorotiseGit will show me a fatal message(close the dialog and try agian, the error keeps shows up):

git.exe update-index [...]

fatal: Unable to create 'D:/CodeRoot/iSource/lfs-test/.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.

using TortoiesGit version TortoiseGit 1.8.16.0, and git version git version 2.6.3.windows.1.