public static void Show(Rect rect, string title, string value, ValidateCallback callback)
        {
            //Close window if already open
            if (window != null)
            {
                Hide();
                return;
            }

            //Open window
            StringPopup.value      = value;
            StringPopup.fieldTitle = title;
            StringPopup.callback   = callback;
            window = CreateInstance <StringPopup>();
            window.ShowPopup();
            window.minSize      = new Vector2(100, 70);
            window.titleContent = new GUIContent("StringFieldPopup");
            window.position     = rect;
        }