WordPress Navigation

You should hook it like this :

function customwidget_init()
{
$args = array(
    'name'          => __( 'Main Navigation', 'theme_text_domain' ),
    'id'            => 'sidebar-navigation',
    'description'   => 'Main Navigation Container',
        'class'         => '',
    'before_widget' => '<div>',
    'after_widget'  => '</div>',
    'before_title'  => '<h2 class="widgettitle">',
    'after_title'   => '</h2>' );
    register_sidebar( $args );
}
add_action( 'widgets_init', 'customwidget_init');

see Codex description