null resolution

src/engine/objects/Mob.java

@@ -1711,7 +1711,7 @@ public class Mob extends AbstractIntelligenceAgent {
                 this.behaviourType = MobBehaviourType.GuardCaptain;
                 this.spawnTime = 60 * 15;
                 this.isPlayerGuard = true;
-                if(ZoneManager.getCityAtLocation(this.building.getLoc()) != null)
+                if(this.building != null && ZoneManager.getCityAtLocation(this.building.getLoc()) != null)
                     this.guardedCity = ZoneManager.getCityAtLocation(this.building.getLoc());
                 else
                     this.guardedCity = null;