For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
return !(empty($formModel->formData['mytable___elementA']) && empty($formModel->formData['mytable___elementB']));
return $data != '';
return !(empty($formModel->formData['mytable ___ elementA']) && empty($formModel->formData['mytable ___ elementB']));