line 200 sets the master key value to use the product_id url variable:
$WA_masterKeyValue = "".$_GET['product_id'] ."";
the error suggests that you are not passing a url variable named product_id in the link to the update page. when you open the update page, what is the full address in the address bar? it should include a precept_id variable, for example:
products_update.php?product_id=212