Xcode 9 commit: Couldn't communicate with helper application

The solution is to add changes once with a git command. Use terminal and navigate to Xcode project folder. If you are in the right folder, this command:

ls -al

will list a .git folder. Then you know you're at the right place. Then execute

git commit -a -m "Commit title here"

After that commit via Xcode should work again.

For me I had to populate Full Name and Email address in Preferences > Source Control > GIT (tab). They were blank.

If you cant change the git name in Xcode, you can change username and email using the terminal:

xcrun git config --global user.name 'new_user_name'
xcrun git config --global user.email '[email protected]'

And restart Xcode.