/// <summary>
        /// Removes a screen from the screen manager. You should normally
        /// use GameScreen.ExitScreen instead of calling this directly, so
        /// the screen can gradually transition off rather than just being
        /// instantly removed.
        /// </summary>
        /// <param name="screen">The added screen.</param>
        public void RemoveScreen(GameScreen screen)
        {
            // If we have a graphics device, tell the screen to unload content.
              if (isInitialized)
              {
            screen.UnloadContent();
              }

              screens.Remove(screen);
              screensToUpdate.Remove(screen);
        }
        /// <summary>
        /// Adds a new screen to the screen manager.
        /// </summary>
        /// <param name="screen">The screen.</param>
        /// <exception cref="ArgumentNullException">screen parameter was null</exception>
        public void AddScreen(GameScreen screen)
        {
            if (screen == null)
              {
            throw new ArgumentNullException("screen");
              }

              screen.ScreenManager = this;
              screen.IsExiting = false;

              // If we have a graphics device, tell the screen to load content.
              if (isInitialized)
              {
            screen.LoadContent();
              }

              screens.Add(screen);
        }