Объединение вопросов Woocommerce Cart и ckeckout

Мне нужно настроить Woocommerce корзина и добавить форму заказа на странице корзины.
Я пытался вставить код из form-checkout.php в cart.php, но он не работает. У меня ошибка:

Неустранимая ошибка: необученная ошибка: вызов функции-члена
get_checkout_fields () в null

Как я понял, я только что вызвал неопределенную функцию, но как я могу ее определить?
Если кто-то знаком с настройкой woocommerce шаблоны, пожалуйста, помогите мне.
Также было бы очень приятно, если бы кто-нибудь поделился полезными ссылками о настройке woocommerceЯ уже много читал об этом, но большинство из них были бесполезны.

Мой код из cart.php ниже.

Заранее спасибо!

<?php
/**
* Cart Page
*
* This template can be overridden by copying it to yourtheme/woocommerce/cart/cart.php.
*
* HOWEVER, on occasion WooCommerce will need to update template files and you
* (the theme developer) will need to copy the new files to your theme to
* maintain compatibility. We try to do this as little as possible, but it does
* happen. When this occurs the version of the template file will be bumped and
* the readme will list any important changes.
*
* @see     https://docs.woocommerce.com/document/template-structure/
* @author  WooThemes
* @package WooCommerce/Templates
* @version 3.1.0
*/
?>
<h2 class="main-h2">
<span class="thin">В </span>
КОРЗИНЕ
</h2>

<? if ( ! defined( 'ABSPATH' ) ) {
exit;
}

wc_print_notices();

do_action( 'woocommerce_before_cart' ); ?>

<form class="woocommerce-cart-form" action="<?php echo esc_url( wc_get_cart_url() ); ?>" method="post">
<?php do_action( 'woocommerce_before_cart_table' ); ?>

<table class="shop_table shop_table_responsive cart woocommerce-cart-form__contents" cellspacing="0">
<thead>
<tr>
<th class="product-remove">&nbsp;</th>
<th class="product-thumbnail">&nbsp;</th>
<th class="product-name"><?php _e( 'Product', 'woocommerce' ); ?></th>
<th class="product-price"><?php _e( 'Price', 'woocommerce' ); ?></th>
<th class="product-quantity"><?php _e( 'Quantity', 'woocommerce' ); ?></th>
<th class="product-subtotal"><?php _e( 'Total', 'woocommerce' ); ?></th>
</tr>
</thead>
<tbody>
<?php do_action( 'woocommerce_before_cart_contents' ); ?>

<?php
foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
$_product   = apply_filters( 'woocommerce_cart_item_product', $cart_item['data'], $cart_item, $cart_item_key );
$product_id = apply_filters( 'woocommerce_cart_item_product_id', $cart_item['product_id'], $cart_item, $cart_item_key );

if ( $_product && $_product->exists() && $cart_item['quantity'] > 0 && apply_filters( 'woocommerce_cart_item_visible', true, $cart_item, $cart_item_key ) ) {
$product_permalink = apply_filters( 'woocommerce_cart_item_permalink', $_product->is_visible() ? $_product->get_permalink( $cart_item ) : '', $cart_item, $cart_item_key );
?>
<tr class="woocommerce-cart-form__cart-item <?php echo esc_attr( apply_filters( 'woocommerce_cart_item_class', 'cart_item', $cart_item, $cart_item_key ) ); ?>">

<td class="product-remove">
<?php
echo apply_filters( 'woocommerce_cart_item_remove_link', sprintf(
'

<a href="%s" class="pi-close-btn" aria-label="%s" data-product_id="%s" data-product_sku="%s">&times;>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" x="0px" y="0px" viewBox="0 0 357 357" style="enable-background:new 0 0 357 357;" xml:space="preserve">
<g>
<g>
<polygon points="357,35.7 321.3,0 178.5,142.8 35.7,0 0,35.7 142.8,178.5 0,321.3 35.7,357 178.5,214.2 321.3,357 357,321.3     214.2,178.5   " fill="#ffffff"/>
</g>
</g>
</svg>
</a>',
esc_url( WC()->cart->get_remove_url( $cart_item_key ) ),
__( 'Remove this item', 'woocommerce' ),
esc_attr( $product_id ),
esc_attr( $_product->get_sku() )
), $cart_item_key );
?>
</td>

<td class="product-thumbnail">
<?php
$thumbnail = apply_filters( 'woocommerce_cart_item_thumbnail', $_product->get_image(), $cart_item, $cart_item_key );

if ( ! $product_permalink ) {
echo $thumbnail;
} else {
printf( '<a href="%s">%s</a>', esc_url( $product_permalink ), $thumbnail );
}
?>
</td>

<td class="product-name" data-title="<?php esc_attr_e( 'Product', 'woocommerce' ); ?>">
<?php
if ( ! $product_permalink ) {
echo apply_filters( 'woocommerce_cart_item_name', $_product->get_name(), $cart_item, $cart_item_key ) . '&nbsp;';
} else {
echo apply_filters( 'woocommerce_cart_item_name', sprintf( '<a href="%s">%s</a>', esc_url( $product_permalink ), $_product->get_name() ), $cart_item, $cart_item_key );
}

// Meta data
echo WC()->cart->get_item_data( $cart_item );

// Backorder notification
if ( $_product->backorders_require_notification() && $_product->is_on_backorder( $cart_item['quantity'] ) ) {
echo '<p class="backorder_notification">' . esc_html__( 'Available on backorder', 'woocommerce' ) . '</p>';
}
?>
</td>

<td class="product-price" data-title="<?php esc_attr_e( 'Price', 'woocommerce' ); ?>">
<?php
echo apply_filters( 'woocommerce_cart_item_price', WC()->cart->get_product_price( $_product ), $cart_item, $cart_item_key );
?>
</td>

<td class="product-quantity" data-title="<?php esc_attr_e( 'Quantity', 'woocommerce' ); ?>">
<?php
if ( $_product->is_sold_individually() ) {
$product_quantity = sprintf( '1 <input type="hidden" name="cart[%s][qty]" value="1" />', $cart_item_key );
} else {
$product_quantity = woocommerce_quantity_input( array(
'input_name'  => "cart[{$cart_item_key}][qty]",
'input_value' => $cart_item['quantity'],
'max_value'   => $_product->get_max_purchase_quantity(),
'min_value'   => '0',
), $_product, false );
}

echo apply_filters( 'woocommerce_cart_item_quantity', $product_quantity, $cart_item_key, $cart_item );
?>
</td>

<td class="product-subtotal" data-title="<?php esc_attr_e( 'Total', 'woocommerce' ); ?>">
<?php
echo apply_filters( 'woocommerce_cart_item_subtotal', WC()->cart->get_product_subtotal( $_product, $cart_item['quantity'] ), $cart_item, $cart_item_key );
?>
</td>
</tr>
<?php
}
}
?>

<?php do_action( 'woocommerce_cart_contents' ); ?>

<tr>
<td colspan="6" class="actions">

<?php if ( wc_coupons_enabled() ) { ?>
<div class="coupon">
<label for="coupon_code"><?php _e( 'Coupon:', 'woocommerce' ); ?></label> <input type="text" name="coupon_code" class="input-text" id="coupon_code" value="" placeholder="<?php esc_attr_e( 'Coupon code', 'woocommerce' ); ?>" /> <input type="submit" class="button" name="apply_coupon" value="<?php esc_attr_e( 'Apply coupon', 'woocommerce' ); ?>" />
<?php do_action( 'woocommerce_cart_coupon' ); ?>
</div>
<?php } ?>

<input type="submit" class="button" name="update_cart" value="<?php esc_attr_e( 'Update cart', 'woocommerce' ); ?>" />

<?php do_action( 'woocommerce_cart_actions' ); ?>

<?php wp_nonce_field( 'woocommerce-cart' ); ?>
</td>
</tr>

<?php do_action( 'woocommerce_after_cart_contents' ); ?>
</tbody>
</table>
<?php do_action( 'woocommerce_after_cart_table' ); ?>
</form>

<div class="cart-collaterals">
<?php
/**
* woocommerce_cart_collaterals hook.
*
* @hooked woocommerce_cross_sell_display
* @hooked woocommerce_cart_totals - 10
*/
do_action( 'woocommerce_cart_collaterals' );
?>
</div>

<?php do_action( 'woocommerce_after_cart' ); ?><?php If ( ! defined( 'ABSPATH' ) ) {
exit;
}

wc_print_notices();

do_action( 'woocommerce_before_checkout_form', $checkout );

// If checkout registration is disabled and not logged in, the user cannot checkout

?>

<?php if ( ! defined( 'ABSPATH' ) ) {
exit;
}

wc_print_notices();

do_action( 'woocommerce_before_checkout_form', $checkout ); ?>

<form name="checkout" method="post" class="checkout woocommerce-checkout" action="<?php echo esc_url( wc_get_checkout_url() ); ?>" enctype="multipart/form-data">

<?php if ( $checkout->get_checkout_fields() ) : ?>

<?php do_action( 'woocommerce_checkout_before_customer_details' ); ?>

<div class="col2-set" id="customer_details">
<div class="col-1">
<?php do_action( 'woocommerce_checkout_billing' ); ?>
</div>

<div class="col-2">
<?php do_action( 'woocommerce_checkout_shipping' ); ?>
</div>
</div>

<?php do_action( 'woocommerce_checkout_after_customer_details' ); ?>

<?php endif; ?>

<h3 id="order_review_heading"><?php _e( 'Your order', 'woocommerce' ); ?></h3>

<?php do_action( 'woocommerce_checkout_before_order_review' ); ?>

<div id="order_review" class="woocommerce-checkout-review-order">
<?php do_action( 'woocommerce_checkout_order_review' ); ?>
</div>

<?php do_action( 'woocommerce_checkout_after_order_review' ); ?>

</form>

<?php do_action( 'woocommerce_after_checkout_form', $checkout ); ?>