public function init()
 {
     parent::init();
     Requirements::css('silverstripe-upgrade-notification/css/custom.css');
     CMSMenu::remove_menu_item('UpgradeNotificationPage');
     $upgrade_notification_page = new UpgradeNotificationPage();
     if (!$upgrade_notification_page->isCurrentVersion() || $upgrade_notification_page->isCurrentVersion() && $upgrade_notification_page->config()->ShowMenuItemWhenCurrent) {
         CMSMenu::add_menu_item($upgrade_notification_page->getCode(), $upgrade_notification_page->getUpgradeMessage(), 'admin/upgrade/', null, 1000);
     }
 }
<?php

LeftAndMain::require_css(basename(__DIR__) . '/css/custom.css');
CMSMenu::remove_menu_item('UpgradeNotificationPage');
$upgrade_notification_page = new UpgradeNotificationPage();
if (!$upgrade_notification_page->isCurrentVersion() || $upgrade_notification_page->isCurrentVersion() && $upgrade_notification_page->config()->ShowMenuItemWhenCurrent) {
    CMSMenu::add_menu_item($upgrade_notification_page->getCode(), $upgrade_notification_page->getUpgradeMessage(), 'admin/upgrade/', null, 1000);
}