Fix CostTapType NPE

forge-game/src/main/java/forge/game/keyword/Hexproof.java

@@ -33,5 +33,4 @@ public class Hexproof extends KeywordInstance<Hexproof> {
         return false;
     }
 
-    
 }

forge-gui/src/main/java/forge/player/HumanCostDecision.java

@@ -1096,7 +1096,7 @@ public class HumanCostDecision extends CostDecisionMakerBase {
             c = AbilityUtils.calculateAmount(source, amount, ability);
         }
 
-        if (c == 0) {
+        if (c != null && c == 0) {
             return PaymentDecision.number(0);
         }