Hi Ryan,
and thank you for this plugin.
There was some changes in WordPress wp-admin/includes/class-walker-nav-menu-edit.php file (at least after version 4.7.) and I think that line 14 in your includes/walker-nav-menu.edit.php should be changed from
$position = stripos( $tempOutput, ‘<p class=”field-move’ );
to
$position = stripos( $tempOutput, ‘<fieldset class=”field-move’ );