I have a module I'm converting to 2.0.16. I'm unable to use $xoopsTpl->assing()

What is the correct sequence of include files I should have in an admin page to enable $xoopsTpl?

My current page is:

$xoopsOption['template_main'] = 'oscmembership_optionlist.html';
//include("../../../mainfile.php");

include '../../../include/cp_header.php';
include '../../../class/xoopsformloader.php';
include '../../../class/pagenav.php';

include_once XOOPS_ROOT_PATH.'/class/xoopsform/grouppermform.php';

xoops_cp_header();

I get this error:
Call to a member function on a non-object in /var/www/xoops2016/modules/oscmembership/admin/osclistselect_smarty.php on line 58

The template engine is not instanciated by default in an admin page, you have to do it yourself.