Send error log to email

$ci is NULL

With that If I didn't get the error, but also It doesn't send any email.

The problem is that get_instance(); return NULL

UPDATE:

If I move get_instance() in __construct() as I saw here: http://www.codeigniter.com/userguide3/ge...asses.html

I receive the following error:
Fatal error: Class 'CI_Controller' not found in system\core\CodeIgniter.php on line 366