anti-NPE measure: http://www.slightlymagic.net/forum/viewtopic.php?p=115101#p115101

src/main/java/forge/game/ai/ComputerUtilCost.java

@@ -336,6 +336,8 @@ public class ComputerUtilCost {
      * @return a boolean.
      */
     public static boolean canPayCost(final SpellAbility sa, final Player player) {
+        sa.setActivatingPlayer(player); // complaints on NPE had came before this line was added.
+
         // Check for stuff like Nether Void
         int extraManaNeeded = 0;
         if (sa instanceof Spell) {