action user_new_form param is a string

use like this:

$id = $user->ID;

And if user is new set $id = 9999

function my_show_extra_profile_fields( $user ) {

    if(is_string($user) === true){
        $user = new stdClass();//create a new
        $id = -9999;
    }
    $newsletter = esc_attr( get_the_author_meta( '_newsletter', $id ) );
    unset($user);
}