You need to add a hidden field to the form that contains the 'id' that must be passed to the page, otherwise the id will no longer be present in the parameters of the page when the form is submitted.
For example (in definition())
$mform->addElement('hidden', 'id', $urlid); $mform->setType('id', PARAM_INT);
Additionally, in Moodle you should not access $_GET directly - use the wrapper functions required_param() or optional_param() because they:
Therefore, your access to $_GET['id'] should be replaced with:
$urlid = optional_param('id', null, PARAM_INT);