//for extending an existing terminal
        public static object ShowPopUp(Terminal terminal)
        {
            PopUpWindow window = new PopUpTerminal(terminal);

            window.ShowDialog();

            return(window.Selected);
        }
        //for creating a new terminal
        public static object ShowPopUp(Airport airport)
        {
            PopUpWindow window = new PopUpTerminal(airport);

            window.ShowDialog();

            return(window.Selected);
        }
        //for extending an existing terminal
        public static object ShowPopUp(Terminal terminal)
        {
            PopUpWindow window = new PopUpTerminal(terminal);
            window.ShowDialog();

            return window.Selected;
        }
        //for creating a new terminal
        public static object ShowPopUp(Airport airport)
        {
            PopUpWindow window = new PopUpTerminal(airport);
            window.ShowDialog();

            return window.Selected;
        }