early exit for pets attacking a null target

src/engine/mobileAI/MobAI.java

@@ -175,6 +175,8 @@ public class MobAI {
     public static void attackMob(Mob mob, Mob target) {
 
         try {
+            if(mob == null || target == null)
+                return;
 
             if (mob.getRange() >= 30 && mob.isMoving())
                 return;