Harden against null

src/engine/gameManager/BuildingManager.java

@@ -343,6 +343,7 @@ public enum BuildingManager {
             Mob hirelingMob = (Mob) hireling;
 
             if (hirelingMob.getParentZone() != null)
+                if (hirelingMob.getParentZone().zoneMobSet.contains(hirelingMob))
                     hirelingMob.getParentZone().zoneMobSet.remove(hireling);
 
         }
@@ -352,6 +353,7 @@ public enum BuildingManager {
             NPC hirelingNPC = (NPC) hireling;
 
             if (hirelingNPC.getParentZone() != null)
+                if (hirelingNPC.getParentZone().zoneNPCSet.contains(hirelingNPC))
                     hirelingNPC.getParentZone().zoneNPCSet.remove(hireling);
 
         }