code: public final void remove(ApplicationWindow window) {
          if (windows.contains(window)) {
              windows.remove(window);
              window.setWindowManager(null);
              setChanged();
              notifyObservers();
          }
      }
docs: Removes the given window from the set of windows managed by this window
  manager. Does nothing is this window is not managed by this window
  manager.
  
  @param window
  the window