Get user ID when order_completed

Hi, I write function:

add_action( 'woocommerce_order_status_completed', 'order_completed');

function order_completed($order_id) {
   
		$user_id = $order->get_user_id(); // or $order->get_customer_id();
		$order = wc_get_order( $order_id );
		$items = $order->get_items();
		
		foreach ( $items as $item ) {
			$product_name = $item->get_name();			
		}  

		 switch ($product_name) {
		case 'FREE':		
		  update_user_meta($user_id, 'ads-available',  get_user_meta( $user_id, 'ads-available', true ) +1 );
		  break;
		  
		case 'VIP':
		  update_user_meta($user_id, 'ads-available',  get_user_meta( $user_id, 'ads-available', true ) +3 );
		  update_user_meta($user_id, 'karusel',  get_user_meta( $user_id, 'karusel', true ) +1 );
		  break;	
	  }

		
    }

And when I change status order from dashboard I have error:

Type error: E_ERROR in row 102 file /var/www/ /data/www/web-design.com/wp-content/plugins/functions.php.  Uncaught Error: Call to a member function get_user_id() on null in /var/www/ /data/www/web-design.com/wp-content/plugins/functions.php:102
Stack trace:
#0 /var/www/ /data/www/web-design.com/wp-includes/class-wp-hook.php(294): order_completed(862)
#1 /var/www/ /data/www/web-design.com/wp-includes/class-wp-hook.php(316): WP_Hook->apply_filters('', Array)
#2 /var/www/ /data/www/web-design.com/wp-includes/plugin.php(484): WP_Hook->do_action(Array)
#3 /var/www/ /data/www/web-design.com/wp-content/plugins/woocommerce/includes/class-wc-order.php(363): do_action('woocommerce_ord...', 862, Object(Automattic\WooCommerce\Admin\Overrides\Order))
#4 /var/www/ /data/www/web-design.com/wp-content/plugins/woocommerce/includes/class-wc-order.php(221): WC_Order->status_transition()
#5 /var/www/ /data/www/web-design.com/wp-content/plugins/woocommerce/includes/class-wc-order.php(333): WC_Order->save()
#6 /var/www/ /data/www/web-design.com/wp-content/plugins/woocommerce/includes/admin/list-tables/class-wc-admin-li

row 102 it is -  $user_id = $order->get_user_id(); // or $order->get_customer_id();

why it happened?

thx