Hello all!
I faced with a same situation In my code. Error has been shown for "addRule" . Sample:
$radioarray[] =& $mform->createElement('radio', 'yesno', '', get_string('yes'), 1);
$radioarray[] =& $mform->createElement('radio', 'yesno', '', get_string('no'), 0);
$mform->addGroup($radioarray, 'radioarr', '', array(' '), false);
$mform->addRule('yesno', get_string('missinanswer', 'enrol_survey'), 'required');
Change third row solved the problem:
$mform->addGroup($radioarray, 'yesno', '', array(' '), false);
(In addGroup and createElement must be the same name)
I will be glad if my example will help someone