fix exception when updating cart using coupons

Author: ajungg

CHANGELOG.md

@@ -6,6 +6,9 @@ All notable changes to this project will be documented in this file.
 ### Added
 - Added support for Google Pay
 
+### Fixed
+- Fixed exceptions when using coupons that affect the whole cart
+
 ## [0.38.1]
 
 ### Updated

core/src/main/java/io/snabble/sdk/ShoppingCartUpdater.java

@@ -202,7 +202,9 @@ private void commitCartUpdate(int modCount, CheckoutApi.SignedCheckoutInfo signe
 
                         if (lineItem.type == CheckoutApi.LineItemType.COUPON) {
                             ShoppingCart.Item refersTo = cart.getByItemId(lineItem.refersTo);
-                            refersTo.setManualCouponApplied(lineItem.redeemed);
+                            if (refersTo != null) {
+                                refersTo.setManualCouponApplied(lineItem.redeemed);
+                            }
                         }
                     }
                 }