private function getFsRoot(): ?string
    {
        if (null === $this->fsRoot) {
            $tempDir = rtrim(sys_get_temp_dir(), '\\/');

            do {
                $name = $tempDir.'/'.uniqid('vfs', true);
            } while (file_exists($name));

            $this->fs->mkdir($name);
            $this->fsRoot = $name;
        }

        return $this->fsRoot;
    }