For version 19.1 and above goto specified directory and delete these mentioned files:
C:\Users\UserName\AppData\Roaming\syntevo\SmartGit\20.1<smart-git-version>
C:\Users\UserName\AppData\Roaming\syntevo\SmartGit\20.1\.backup
For the previous version goto specified directory and delete mentioned file:
C:\Users\UserName\AppData\Roaming\syntevo\SmartGit\17<smart-git-version>