this is because of your error reporting level.
your error reporting level is set to include notices.
in reality, this is not really an error that indicates a problem. it is a notice that on page load, there is a variable being referenced that does not exist.
in a production environment, the reporting level should not show notices, and this message will not have any effect.
if you really must make it go away, you can change the line:
$WA_masterKeyValue = "".$_SESSION['WADA_Insert_items'] ."";
$WA_masterKeyValue = "".(isset($_SESSION['WADA_Insert_items'])?$_SESSION['WADA_Insert_items']:"") ."";