Add image to menu item with ACF

get_field() gets the field for the current post in the loop, but the loop isn’t used for menu items, so you need to specify which post you want to get the value from. In the case of menu items, each nav menu item is a WP_Post object, so you want to pass that:

foreach( $items as &$item ) {
    $icon = get_field( 'icon', $item );
    // etc.
}