The fix that I gave you should not call mysqli_errno() if the resource is empty, therefore you should not be getting the message
Warning: mysqli_errno() expects parameter 1 to be mysqli, null given
When I try to force a connection error on my PC I always get a meaningful error message, so something must be seriously wrong with your installation. I can only suggest that you debug the connect() method inside dm.mysqli.class.inc to see what is causing this error.
I also suggest that you try using the latest version of Radicore.