Update ui_extensions.py

    try:

        shutil.rmtree(tmpdir, True)

        repo = git.Repo.clone_from(url, tmpdir)

        with git.Repo.clone_from(url, tmpdir) as repo:

            repo.remote().fetch()

        try:

            for submodule in repo.submodules:

                submodule.update()

        os.rename(tmpdir, target_dir)

        except OSError as err:

            # TODO what does this do on windows? I think it'll be a different error code but I don't have a system to check it

            # Shouldn't cause any new issues at least but we probably want to handle it there too.

            if err.errno == errno.EXDEV:

                # Cross device link, typical in docker or when tmp/ and extensions/ are on different file systems

                # Since we can't use a rename, do the slower but more versitile shutil.move()

                shutil.move(tmpdir, target_dir)

            else:

                # Something else, not enough free space, permissions, etc.  rethrow it so that it gets handled.

                raise(err)

        import launch

        launch.run_extension_installer(target_dir)