Автоматическое добавление артикула в VirtueMart 3 - [VM2&3] Общие вопросы и ошибки

<td height="18">
<input type="text" class="inputbox" name="product_sku" id="product_sku" value="<?php echo $this->product->product_sku; ?>" size="32" maxlength="255" />
</td>

<td height="18">
<!--<input type="text" class="inputbox" name="product_sku" id="product_sku" value="<?php echo $this->product->product_sku; ?>" size="32" maxlength="255" /> -->
<?php  //AUTO product_sku
    echo '<input type="text" class="inputbox" name="product_sku" id="product_sku" value="';
          if ($this->product->product_sku) $val_sku= $this->product->product_sku;
                    else {
                        $db_sku = JFactory::getDBO();
                        $q_sku = 'SELECT `product_sku` FROM #__virtuemart_products ORDER BY `product_sku` DESC LIMIT 1';
                        $db_sku->setQuery($q_sku);
                        $db_sku->next_record();
                        $val_sku = intval($this->product->virtuemart_product_id)+rand(intval($this->product->product_id), 999999);
                          }
    echo $val_sku.'" size="32" maxlength="64" />';
?>
</td>

Notice: Undefined property: TableProducts::$product_id in /administrator/components/com_virtuemart/views/product/tmpl/product_edit_information.php on line 74948085

$val_sku = intval($this->product->virtuemart_product_id)+rand(intval($this->product->product_id), 999999); 

$val_sku = intval($this->product->virtuemart_product_id)+rand(intval($this->product->product_sku), 999999);