public static void ShowSceneLocator()
        {
            if (instance != null)
            {
                return;
            }

            var window = CreateInstance <SceneLocator>();

            instance = window;
            window.ShowPopup();

            var size   = new Vector2(300, 300);
            var center = new Vector2(Screen.currentResolution.width / 2 - size.x / 2, Screen.currentResolution.height / 2 - size.y / 2);

            window.position = new Rect(center, size);
            window.Focus();
        }
 private void OnDestroy()
 {
     instance = null;
 }