Dropbox Backup restricted for non-admin users

CustomButtons.php

<?php
// Copyright 1999-2014. Parallels IP Holdings GmbH. All Rights Reserved.
class Modules_DropboxBackup_CustomButtons extends pm_Hook_CustomButtons
{
    public function getButtons()
    {
        if (!pm_Session::getClient()->isAdmin()) {
            return [];
        }

        return [[
            'place' => [self::PLACE_DOMAIN],
            'title' => pm_Locale::lmsg('buttonTitle'),
            'description' => pm_Locale::lmsg('buttonDescription'),
            'link' => pm_Context::getBaseUrl() . '?redirect=domain',
            'icon' => pm_Context::getBaseUrl() . 'images/dropbox.png',
        ]];
    }
}

index.php

<?php
// Copyright 1999-2014. Parallels IP Holdings GmbH. All Rights Reserved.
$moduleId = basename(dirname(__FILE__));

pm_Context::init($moduleId);
Modules_DropboxBackup_Dropbox::init();

if (!pm_Session::getClient()->isAdmin()) {
    throw new pm_Exception('Permission denied');
}

$application = new pm_Application();
$application->run();

/usr/local/psa/admin/htdocs/modules/dropbox-backup/index.php
/usr/local/psa/admin/plib/modules/dropbox-backup/hooks/CustomButtons.php