...
private List<string> _items;

public void Startup() {
   Debug.Log("Uruchamianie menedżera magazynu...");
   _items = new List<string>();    
   status = ManagerStatus.Started;
}

private void DisplayItems() {       
   string itemDisplay = "Przedmioty: ";
   foreach (string item in _items) {
      itemDisplay += item + " ";
   }
   Debug.Log(itemDisplay);
}

public void AddItem(string name) {    
   _items.Add(name);
   DisplayItems();
}
...
