This is a notice, not an error and can be ignored, it is only showing because we have added the code to display all errors and notices.
when you load the page initially, it is looking for the WADA_Insert_user session variable, but the insert has not occurred, so that variable has not been created.
Since this is a Notice, not an Error, it will not effect the page operation.
you can change this line:
error_reporting(E_ALL ^ E_NOTICE);
in order to suppress displaying the notices if you like.