WooCommerce Admin Custom Order Total calculate error

It works well. But there is a problem here. This error message shows when the advance payment field is empty – “A non-numeric value encountered …” . How can this be solved?

add_action( ‘manage_shop_order_posts_custom_column’ , array(&$this,’add_custom_column_content’), 11, 2 );

add_action( ‘woocommerce_admin_order_totals_after_discount’, array(&$this, ‘vp_add_sub_total’), 10, 1);

add_action( ‘woocommerce_process_shop_order_meta’, array(&$this, ‘save_order_custom_field_meta_data’), 12, 2 );

// Output a custom editable field in backend edit order pages under general section

function editable_order_custom_field( $order ) {

        // Get "Delivery Type" from meta data (not item meta data)
        $updated_advance_payment = $order->get_meta('_advance_payment');
    
        // Replace "Delivery Type" value by the meta data if it exist
        $advancePayment = $updated_advance_payment ? $updated_advance_payment : ( isset($item_value) ? $item_value : '');
    
        // Display the custom editable field
        woocommerce_wp_text_input( 
            array(
                'id'            => 'advance_payment',
                'label'         => __("Advance Payment:", "woocommerce"),
                'value'         => $advancePayment,
                'wrapper_class' => 'form-field-wide',
            )
        );
    }

// Save the custom editable field value as order meta data and update order item meta data
function save_order_custom_field_meta_data( $post_id, $post ){
if( isset( $_POST( ‘advance_payment’ ) )){
update_post_meta( $post_id, ‘_advance_payment’, sanitize_text_field( $_POST( ‘advance_payment’ ) ) );

            // Update the existing item meta data
            if( isset( $_POST( 'item_id_ref' ) ) ){
                wc_update_order_item_meta( $_POST( 'item_id_ref' ), 'Advance Payment', $_POST( 'advance_payment' ) );
            }
        }
    }

//Display Advance Payment and calculate
function vp_add_sub_total( $the_order ) {
global $post, $the_order;
if ( empty( $the_order ) || $the_order->get_id() !== $post->ID ) {
$the_order = wc_get_order( $post->ID );
}
?>

        <tr>
        <td class="label">Advance Payment:</td>
        <td width="1%"></td>
        <td class="total"><?php echo wc_price(get_post_meta($post->ID, "_advance_payment", true));?></td>
        </tr>
        
        <?php

        $getTotal = $the_order->get_total();

        $updateTotal = $getTotal - get_post_meta($post->ID, "_advance_payment", true);

        $the_order->set_total($updateTotal);

        $the_order->save();

    }