Undefined offset catalog/model/total/coupon.php on line 144

Hi,

I'm using OpenCart 2.2.0.0 with "TheKitchen" template and X-Shipping extension the free not the pro version.
Under Advertisement->Coupon we added a -10% coupon.

When I'm using this coupon in the order process and i reach the Payment-Method page. On the top of the page it shows me the error that in coupon.php on line 144 an undefined offset is tried to accessed. (see attachement)
If I generate the same order without using the coupon-code, also the error on this page is not present.

Can you please help me?
Thanks in advance.
With kind regards.
RG

As per your opencart version and file name on which error is coming. It seems it is related with tax class for this product.
Please check if tax has been properly added in backend for product.
Also check tax class and tax rate in system - localisation.