Fix guard archers / immobile ranged NPCs rare null reference exception when their target breaks LoS or gets out of range

Author: bm01

GameServer/ECS-Components/Actions/NpcAttackAction.cs

@@ -45,7 +45,10 @@ public override bool OnOutOfRangeOrNoLosRangedAttack()
             {
                 GameObject oldTarget = _target;
                 StandardMobBrain brain = _npcOwner.Brain as StandardMobBrain;
-                brain.RemoveFromAggroList(_losCheckTarget as GameLiving);
+
+                if (_losCheckTarget is GameLiving livingLosCheckTarget)
+                    brain.RemoveFromAggroList(livingLosCheckTarget);
+
                 brain.AttackMostWanted(); // This won't immediately start the attack on the new target, but we can use `TargetObject` to start checking it.
                 GameObject newTarget = _npcOwner.TargetObject;