unregister_sidebar() in stand-alone theme not working

Thanks to Milo and G.M. for pointing to the master key, as G.M. said:

you have to use a lower priority than one register_sidebar runs, but
lower priority means higher number.

So the final code that is working:

function site_unregister_sidebar() {

    if ( is_admin() && current_user_can('editor') ) {
        unregister_sidebar( 'my_custom_widget_area' );
    }
}

add_action('widgets_init', 'site_unregister_sidebar', 20);

Because I set my sidebar with priority of 10 with the same hook – widgets_init.

function theme_widgets_init() {

    register_sidebar( array (
        'name' => 'Custom Widget Area',
        'id' => 'my_custom_widget_area'
    ) );
}

add_action( 'widgets_init', 'theme_widgets_init', 10 );