public void handleClosedUser(String username) {
        if (usernameToChats.get(username).isVisible()) {
            usernameToChats.get(username).dispose();
            usernameToChats.remove(username);
            JOptionPane.showMessageDialog(frame, "The user "+username+" is " +
            		"no longer online.", "Log Off Notification",
            		JOptionPane.INFORMATION_MESSAGE);
        } //otherwise do nothing
    }