If your SourceTree complains on startup saying that 'git status' failed with code 128: fatal: Not a git repository (or any of the parent directories): .git (Here some folder) most likely the problem is that you have deleted a repository while tab with the repository was opened. To solve this navigate to
"%USERPROFILE%\AppData\Local\Atlassian\SourceTree\opentabs.xml",
open it with any text editor and remove line pointing to the repository SourceTree complains about. Save the file and restart SourceTree, the problem should disappear.