OOP and WordPress plugin theme supporting

I'm trying to create the WordPress plug-in using OOP and I got stuck at the beginning. Will be faster if I show code than to describe the problem.

It works:

class Plugin{
    public function __construct(){
        return;
    }
 }
 function customHeader(){
     add_theme_support('custom-header');
 }
 add_action( 'after_setup_theme', 'customHeader' );

But it doesn't:

class Plugin{
    public function __construct(){
        if (is_admin()) {
           add_action('admin_init', array($this,'admin_init'));
        }
    }
    public function admin_init(){
        add_action( 'after_setup_theme', array($this,'customHeader') );
    }
    public function customHeader(){
        add_theme_support('custom-header');
    }
}
$plugin=new Plugin();

Do you have any idea why it doesn't work and where there is a mistake ?

via Chebli Mohamed