Hi,
Edit file edit_mega_menu_walker.php, insert
do_action( 'wp_nav_menu_item_custom_fields', $item_id, $item, $depth, $args );
at the line #467, just after the ‘description’ field paragraph, like this:
<p class="field-move hide-if-no-js description description-wide">
...
</p>
<?php
do_action( 'wp_nav_menu_item_custom_fields', $item_id, $item, $depth, $args );
?>
I hope it will help. In case of failure, I will need access to a full theme copy, to make own tests.