Undefined property: plgHikashoppaymentExample::$ur

I am developing a payment method using the Example.zip files as guide. Everything works well until I uncommented the following two line:

$return_url = HIKASHOP_LIVE.'index.php?option=com_hikashop&ctrl=checkout&task=after_end&order_id='.$order_id.$this->url_itemid;

$this->app->redirect($return_url);

Then i get this message:

Notice: Undefined property: plgHikashoppaymentExample::$url_itemid in C:\Bitnami\wampstack-5.6.22-0\apache2\htdocs\Joomla_3.5.1\plugins\hikashoppayment\example\example.php on line 275

Notice: Undefined property: plgHikashoppaymentExample::$app in C:\Bitnami\wampstack-5.6.22-0\apache2\htdocs\Joomla_3.5.1\plugins\hikashoppayment\example\example.php on line 282

Fatal error: Call to a member function redirect() on null in C:\Bitnami\wampstack-5.6.22-0\apache2\htdocs\Joomla_3.5.1\plugins\hikashoppayment\example\example.php on line 282

How can I clear the error so that I can proceed with the development. Thanks

Hello,
Thank you for the quick response. Attached is the file containing the code.

Thank you.

Thanks a million! Including those lines solved the problem.
Have a nice day.