void Awake ()
    {
        saveDirPath = Application.dataPath + "/Levels/";

        Tools.Assert (buttonSave != null, "buttonSave is not assigned");
        UIEventListener.Get (buttonSave.gameObject).onClick += OnButtonSave_Click;

        Tools.Assert (buttonLoad != null, "buttonLoad is not assigned");
        UIEventListener.Get (buttonLoad.gameObject).onClick += OnButtonLoad_Click;

        Tools.Assert (buttonInputDialogOk != null, "buttonInputDialogOk is not assigned");
        UIEventListener.Get (buttonInputDialogOk.gameObject).onClick += OnButtonInputDialogOk_Click;

        Tools.Assert (buttonInputDialogCancel != null, "buttonImputDialogCancel is not assigned");
        UIEventListener.Get (buttonInputDialogCancel.gameObject).onClick += OnButtonInputDialogCancel_Click;

        Tools.Assert (buttonselectLevelDialogClose != null, "buttonselectLevelDialogClose is not assigned");
        UIEventListener.Get (buttonselectLevelDialogClose.gameObject).onClick += OnButtonselectLevelDialogClose_Click;
    }

//in Tools class
    public static void Assert (bool condition, string message)
    {
#if UNITY_EDITOR
        if (!condition) {
            throw new UnityException (message);
        }
#endif
    }