Also, skrill_orders.php on admin pages instantiates a new order class on line 162

Code:
  $order = new order($skrillGwayInfo->zc_oid);
and obviously with a NULL order_id input. This causes inside the cart method in 274 to trow an error for currencies->get_decimal_places

Code:
  $decimals = $currencies->get_decimal_places($_SESSION['currency']);
I reckon, there is no session currency definition for the admin area.
How to overcome this error?

The exact PHP error message is like this:

[30-Jan-2019 11:36:51 UTC] PHP Fatal error: Uncaught Error: Call to a member function get_decimal_places() on null in /httpd.www/includes/classes/order.php:277
Stack trace:
#0 /httpd.www/includes/classes/order.php(49): order->cart()
#1 /httpd.www/admin/skrill_orders.php(162): order->__construct(NULL)
#2 {main}
thrown in /httpd.www/includes/classes/order.php on line 277