Attempting to use the helm-marked-files-in-dired action fails when called from helm-source-ls-git, as the value of helm-ff-default-directory is nil. Advising helm-ls-git-ls to let-bind the value of helm-ff-default-directory to (helm-ls-git-root-dir) works as a temporary fix.
To reproduce:
Navigate to a valid project and call helm-ls-git-ls
Navigate to the helm-source-ls-git source named "Git files"
Mark 0 or more files
From select action menu, select "Marked files in dired"