void OnTriggerEnter(Collider collider)
 {
     if (collider.GetComponent <WalkingUnit>() && collider.GetComponent <WalkingUnit>().HasInventory())
     {
         NearsWalkingUnits.Add(collider.GetComponent <WalkingUnit>());
     }
 }
 void OnTriggerExit(Collider collider)
 {
     if (collider.GetComponent <WalkingUnit>() && collider.GetComponent <WalkingUnit>().HasInventory())
     {
         NearsWalkingUnits.Remove(collider.GetComponent <WalkingUnit>());
     }
 }