Uncaught Error: Call to a member function generate_cart_id() on null

Question

I have the following code that redirect to WooCommerce checkout page when adding a product to cart instead of redirecting to cart page. It works nicely.

// Immediately go to checkout when adding to cart.
add_filter( 'woocommerce_add_to_cart_redirect', function( $url ) {
    return wc_get_checkout_url();
});

add_filter( 'woocommerce_product_add_to_cart_url', 'custom_fix_for_individual_products', 10, 2 );
function custom_fix_for_individual_products( $add_to_cart_url, $product ){
    $product_cart_id = WC()->cart->generate_cart_id( $product->get_id() );
    if( 
        $product->get_sold_individually() // if individual product
        && !is_null($product_cart_id) // Added to try and understand the error I'm getting
        // The next line causes a server error, but only in the edit view of the Homepage after clicking on "Update".
        && WC()->cart->find_product_in_cart( $product_cart_id ) // if already in the cart.
        && $product->is_purchasable()
    ) {
        $add_to_cart_url = wc_get_checkout_url(); // Instead of adding it to cart, go to checkout.
    }

    return $add_to_cart_url;
}

However in backend, when I edit the homepage and click on Update, I get a server error that points to the 2nd function code.

This is what I get on the server log:

PHP Fatal error:  Uncaught Error: Call to a member function generate_cart_id() on null in /home/xxx/public_html/wp-content/plugins/code-snippets/php/snippet-ops.php(446) : eval()'d code:11
Stack trace:
#0 /home/xxx/public_html/wp-includes/class-wp-hook.php(287): misha_fix_for_individual_products()
#1 /home/xxx/public_html/wp-includes/plugin.php(212): WP_Hook->apply_filters()
#2 /home/xxx/public_html/wp-content/plugins/learndash-woocommerce/learndash_woocommerce.php(1285): apply_filters()
#3 /home/xxx/public_html/wp-content/plugins/woocommerce/includes/class-wc-shortcodes.php(398): WC_Product_Course->add_to_cart_url()
#4 /home/xxx/public_html/wp-includes/shortcodes.php(343): WC_Shortcodes::product_add_to_cart_url()
#5 [internal function]: do_shortcode_tag()
#6 /home/xxx/public_html/wp-includes/shortcodes.php in /home/xxx/public_html/wp-content/plugins/code-snippets/php/snippet-ops.php(446) : eval()'d code on line 11

Any idea how to solve this?

Answer 1

You can use the following that will avoid the error produced on backend:

add_filter( 'woocommerce_add_to_cart_redirect', 'wc_get_checkout_url' );

add_filter( 'woocommerce_product_add_to_cart_url', 'custom_fix_for_individual_products', 10, 2 );
function custom_fix_for_individual_products( $add_to_cart_url, $product ){
    if( is_admin() || current_user_can('edit_pages') ) {
        return $add_to_cart_url; // Exit if it's on admin
    }
    
    $cart_id = WC()->cart->generate_cart_id( $product->get_id() );
    
    if( $product->get_sold_individually() && $product->is_purchasable() 
    && WC()->cart->find_product_in_cart( $cart_id ) ) {
        $add_to_cart_url = wc_get_checkout_url(); 
    }

    return $add_to_cart_url;
}

Code goes in functions.php file of the active child theme (or active theme). It should solve the error.