in function uc_stock_notify_action_email we have a code

$nid = db_result(db_query("SELECT n.nid FROM {uc_products} p INNER JOIN {node} n ON p.vid = n.vid WHERE p.model = '%s'", $model));

is not correct when we are using a multi sku, for example
I have a sku 555 and for attributes i added sku 555d 555s
in this example $model will be 555d or 555s and sql request returned NULL
therefore we should rewrite request for example

  $nid = db_result(db_query("SELECT n.nid FROM {uc_product_stock} n WHERE sku = '%s'", $model));