How to fix "Uncaught Error: Call to a member function get_tax_class() on null" in WooCommerce?

I’ve an ecommerce app that’s was working fine some days ago. After I update the wordpress and woocommerce plugin, the app suddenly stopped working.

I think that’s something about the plugin that I use (mstore app-mobile), but unfortunately is not possible to update this plugin.

The error happen on this updateCartQty function. My guess is that something about the function parameter.

public function updateCartQty()
{        
   $cart_item_key = $_REQUEST['key'];
   $qty = (int)$_REQUEST['quantity'];

   global $woocommerce;
   $woocommerce->cart->set_quantity($cart_item_key, $qty);   
   $this->cart();    
}

This is the log file:

2021-02-22T23:43:48+00:00 CRITICAL Uncaught Error: Call to a member function get_tax_class() on null in /var/www/html/ecommerce/htdocs/wp-content/plugins/woocommerce/includes/class-wc-cart-totals.php:225
Stack trace:
#0 /var/www/html/ecommerce/htdocs/wp-content/plugins/woocommerce/includes/class-wc-cart-totals.php(651): WC_Cart_Totals->get_items_from_cart()
#1 /var/www/html/ecommerce/htdocs/wp-content/plugins/woocommerce/includes/class-wc-cart-totals.php(143): WC_Cart_Totals->calculate_item_totals()
#2 /var/www/html/ecommerce/htdocs/wp-content/plugins/woocommerce/includes/class-wc-cart-totals.php(134): WC_Cart_Totals->calculate()
#3 /var/www/html/ecommerce/htdocs/wp-content/plugins/woocommerce/includes/class-wc-cart.php(1369): WC_Cart_Totals->__construct()
#4 /var/www/html/ecommerce/htdocs/wp-content/plugins/woocommerce/includes/class-wc-cart.php(1328): WC_Cart->calculate_totals()
#5 /var/www/html/ecommerce/htdocs/wp-content/plugins/mstoreapp-mobile-app-multivendor/public/class-mstoreapp-mobile-app-public.php(1277): WC_Cart->set_quantity()
#6 , em /var/www/html/ecommerce/htdocs/wp-content/plugins/woocommerce/includes/class-wc-cart-totals.php, na linha 225

System informations:

WC Version: 5.0.0
REST API Version: ✔ 5.0.0
WC Blocks Version: ✔ 4.0.0
Action Scheduler Version: ✔ 3.1.6
WC Admin Version: ✔ 1.9.0
Log Directory Writable: ✔
WP Version: 5.6.2
WP Multisite: –
WP Memory Limit: 256 MB
WP Debug Mode: –
WP Cron: ✔
Language: pt_BR
External object cache: –

### Server Environment ###

Server Info: Apache/2.4.18 (Ubuntu)
PHP Version: 7.4.15
PHP Post Max Size: 8 MB
PHP Time Limit: 30
PHP Max Input Vars: 1000
cURL Version: 7.47.0
OpenSSL/1.0.2g

SUHOSIN Installed: –
MySQL Version: 8.0.20
Max Upload Size: 2 MB
Default Timezone is UTC: ✔
fsockopen/cURL: ✔
SoapClient: ✔
DOMDocument: ✔
GZip: ✔
Multibyte String: ✔
Remote Post: ✔
Remote Get: ✔

### Database ###

WC Database Version: 5.0.0

### Active Plugins (13) ###

Redux: por Redux.io + Dovy Paukstys – 4.1.24
Akismet Anti-Spam: por Automattic – 4.1.8
All in One SEO: por Equipe All in One SEO – 4.0.15
Google Analytics for WordPress by MonsterInsights: por MonsterInsights – 7.16.2
Jetpack por WordPress.com: por Automattic – 9.4
Monetization Code plugin: por aerin Singh – 1.0
Mstoreapp Mobile Multivendor: por Mstoreapp – 9.0.1
Simple Tags: por WebFactory Ltd – 2.63
WCFM - WooCommerce Frontend Manager: por WC Lovers – 6.5.6
WCFM - WooCommerce Multivendor Marketplace: por WC Lovers – 3.4.6
WCFM - WooCommerce Multivendor Membership: por WC Lovers – 2.9.5
WooCommerce: por Automattic – 5.0.0
WP Mail SMTP: por WPForms – 2.6.0

### Inactive Plugins (0) ###

What can I do to solve this error? I appreciate any help. Thank you for your time.

EDIT 1: I’ve changed the $_REQUEST to $_POST and set the debug on $cart_item_key = $_REQUEST[‘key’];

**File:** wp-content/plugins/mstoreapp-mobile-app-multivendor/public/class-mstoreapp-mobile-app-public.php(1276): WC_Cart->set_quantity()

Function

public function updateCartQty() {

        $cart_item_key = $_POST['key'];
        $qty = (int)$_POST['quantity'];

        echo "<script>console.debug( "PHP DEBUG: $cart_item_key" );</script>";
    
        global $woocommerce;
        $woocommerce->cart->set_quantity($cart_item_key, $qty);
        $this->cart();    
    }

OUTPUT:

<script>console.debug( "PHP DEBUG: 120705de7e61c5b322ad798b8ef225a7" );</script>

EDIT 2:

File: wp-content/plugins/mstoreapp-mobile-app-multivendor/public/class-mstoreapp-mobile-app-public.php(1276): WC_Cart->set_quantity()

I’ve set the array position but error still persists:

$cart_item_key = $_POST['key'][0];
$qty = (int)$_POST['quantity'][0];

OUTPUT

EDIT 3:

File: wp-content/plugins/mstoreapp-mobile-app-multivendor/public/class-mstoreapp-mobile-app-public.php(1276): WC_Cart->set_quantity()

Using var_dump($_POST):

public function updateCartQty() {
   
        $cart_item_key = $_POST['key'][0];
        $qty = (int)$_POST['quantity'][0];
          
        var_dump($_POST);

        global $woocommerce;
        $woocommerce->cart->set_quantity($cart_item_key, $qty);

        $this->cart();

    }

The output:

array(6) {
  ["key"]=>
  string(32) "431cfe4bd4a84b68398e14af4be0bdc3"
  ["quantity"]=>
  string(1) "3"
  ["update_cart"]=>
  string(11) "Update Cart"
  ["_wpnonce"]=>
  string(10) "52e75170fc"
  ["lang"]=>
  string(2) "en"
  ["mstoreapp"]=>
  string(1) "1"
}

EDIT 4:

**File:** wp-content/plugins/woocommerce/includes/class-wc-cart-totals.php

protected function get_items_from_cart() {
                $this->items = array();

                foreach ( $this->cart->get_cart() as $cart_item_key => $cart_item ) {
                        $item                          = $this->get_default_item_props();
                        $item->key                     = $cart_item_key;
                        $item->object                  = $cart_item;
                        $item->tax_class               = $cart_item['data']->get_tax_class();
                        $item->taxable                 = 'taxable' === $cart_item['data']->get_tax_status(); // LINE 225
                        $item->price_includes_tax      = wc_prices_include_tax();
                        $item->quantity                = $cart_item['quantity'];
                        $item->price                   = wc_add_number_precision_deep( $cart_item['data']->get_price() * $cart_item['quantity'] );
                        $item->product                 = $cart_item['data'];
                        $item->tax_rates               = $this->get_item_tax_rates( $item );
                        $this->items[ $cart_item_key ] = $item;
                }
        }

EDIT 5:
as suggested by @Vincenzo Di Gaetano, checking the cart_item_key before increment has solved the problem.

global $woocommerce; if ( $woocommerce->cart->get_cart_item(
$cart_item_key ) ) {
$woocommerce->cart->set_quantity( $cart_item_key, $qty ); }