Uncaught Error: Call to a member function get_cart() on null - woocommerce

Question

On Woocommerce, I have an error message on my function but i don't unserstand why it happens.
Uncaught Error: Call to a member function get_cart() on null in ...
The function checks id a category product is in the cart.
The error is displayed in the order detail in backend
It's used in functions.php , executed on checkout page,
called by these hooks:
woocommerce_before_order_notes
woocommerce_checkout_process
woocommerce_checkout_update_order_meta
woocommerce_admin_order_data_after_billing_address
woocommerce_email_order_meta_keys
https://i.stack.imgur.com/v7Atp.png
function is_in_the_cart() {
$found=false;
// Find if product is in the cart price <=40
foreach( WC()->cart->get_cart() as $cart_item_key => $values )
{
$cart_product = $values['data'];
$price = accessProtected($values['data'], 'changes')['price'];
if ($price >=40 && $cart_product->id=='969') {$found=true;}
}
if ( $found ) {
return true;
} else {
return false;
}
}
add_action('woocommerce_checkout_process', 'wps_select_checkout_field_process');
function wps_select_checkout_field_process() {
if (is_in_the_cart())
{
global $woocommerce;
// Check if set, if its not set add an error.
if ($_POST['dvd'] == "blank")
wc_add_notice( '<strong>Merci de séléctionner un DVD</strong>', 'error' );
}
}
//* Update the order meta with field value
add_action('woocommerce_checkout_update_order_meta', 'wps_select_checkout_field_update_order_meta');
function wps_select_checkout_field_update_order_meta( $order_id ) {
if (is_in_the_cart())
{
if ($_POST['dvd']) update_post_meta( $order_id, 'dvd', esc_attr($_POST['dvd']));
}
}
add_action( 'woocommerce_admin_order_data_after_billing_address', 'wps_select_checkout_field_display_admin_order_meta', 10, 1 );
function wps_select_checkout_field_display_admin_order_meta($order){
if (is_in_the_cart())
{
echo '<p><strong>DVD: </strong> ' . get_post_meta( $order->id, 'dvd', true ) . '</p>';
}
}
//* Add selection field value to emails
add_filter('woocommerce_email_order_meta_keys', 'wps_select_order_meta_keys');
function wps_select_order_meta_keys( $keys ) {
if (is_in_the_cart())
{
$keys['Dvd:'] = 'dvd';
return $keys;
}
}

Answer 1

I think you want to check to make sure the cart isn't empty before your function.
Also... You can access price by using get_price() from the product object which you are retrieving with $values['data']
function is_in_the_cart(){
// Make sure it's only on front end
if (is_admin()) return false;
$found = false;
// If cart is empty - bail and return false
if (empty (WC()->cart->get_cart())) {
return false;
} else {
foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
$cart_product = $values[ 'data' ];
// Find if product is in the cart price <=40
$price = floatval($cart_product->get_price());
if ( $price >= 40 && $cart_product->id == '969' ) {
$found = true;
}
}
if ( $found ) {
return true;
} else {
return false;
}
}
}
UPDATED - added is_admin to prevent from running on backend.