protected function validateSkuCart()
    {
        if ($this->isError('store_id')) {
            return null;
        }

        $field = 'sku';
        $value = $this->getSubmitted($field);

        if (!isset($value)) {
            return null;
        }

        $label = $this->translation->text('SKU');

        if (empty($value)) {
            $this->setErrorRequired($field, $label);
            return false;
        }

        $store_id = $this->getSubmitted('store_id');
        $product = $this->product->getBySku($value, $store_id);

        if (empty($product['product_id'])) {
            $this->setErrorUnavailable($field, $label);
            return false;
        }

        $this->setSubmitted('product', $product);
        return true;
    }