Are you annoyed by the "missing or unexpected file" warning in your ACP? The solution below, works on XF 2.1.8 (and in our experience also for most XF 2.1.x versions) flawless. It doesn't change the functionality, but the ACP looks a little cleaner that way .
1. Go to your FTP 2. Go to src/XF/Admin/Controller 3. Open index.php