if (getCallback() == null) { setCallback(new PageCallback(ManageUsers.PAGE)); }
    public void pageBeginRender(PageEvent event) {        if (getUser() == null) {            setUser(new User());        }                // If the user clicks through to the Extension Pools page, clicking OK or Cancel on that        // page should send the user back here        ICallback callback = new PageCallback(PAGE);        ExtensionPoolsPage poolsPage = (ExtensionPoolsPage) event.getRequestCycle().getPage(ExtensionPoolsPage.PAGE);        poolsPage.setCallback(callback);    }