PHP Notice:  Undefined index: customer_group_id

PHP Notice:  Undefined index: customer_group_id in /home/xxxxxxx/xxxxxxx/xxxxxxx/public_html/catalog/model/extension/module/order_email.php on line 21

This error appears when the ocmod modification file has not added the required modification code.

1. Go to Extensions > Modifications
2. Ensure you have Order Status Email Template in Modification list
3. Click on Refresh modification button on the right top