Harden against null

src/engine/gameManager/BuildingManager.java

@@ -343,7 +343,8 @@ public enum BuildingManager {
             Mob hirelingMob = (Mob) hireling;
 
             if (hirelingMob.getParentZone() != null)
-                hirelingMob.getParentZone().zoneMobSet.remove(hireling);
+                if (hirelingMob.getParentZone().zoneMobSet.contains(hirelingMob))
+                    hirelingMob.getParentZone().zoneMobSet.remove(hireling);
 
         }
 
@@ -352,7 +353,8 @@ public enum BuildingManager {
             NPC hirelingNPC = (NPC) hireling;
 
             if (hirelingNPC.getParentZone() != null)
-                hirelingNPC.getParentZone().zoneNPCSet.remove(hireling);
+                if (hirelingNPC.getParentZone().zoneNPCSet.contains(hirelingNPC))
+                    hirelingNPC.getParentZone().zoneNPCSet.remove(hireling);
 
         }