Based on answer from John Paul Balandan on codeigniterchat.slack:
It is necessary to extending Admin\Config\Pager from Config\Pager, not simply extending from BaseConfig.
Class Pager in config file admin/Config/Pager.php, must be defined as follows:
Code:
namespace Admin\Config;
use Config\Pager as BasePager;
class Pager extends BasePager
{
// ...
}