在WordPress插件中使用zebra_form时出错

我正在尝试在元框中使用Zebra_form，用于我正在处理的Wordpress插件。

简单起见，我将代码简化为：

public function render_metabox( $post ) {
    // instantiate a Zebra_Form object
    $form = new Zebra_Form('form');
    // the label for the "email" field
    $form->add('label', 'label_email', 'email', 'Email');
    // add the "email" field
    $obj = $form->add('text', 'email', '', array('autocomplete' => 'off'));
    // auto generate output, labels above form elements
    $form->render();
}

但是当我尝试运行它时，我收到以下错误：

Warning: Cannot modify header information - headers already sent by (output started at /Users/dp/Documents/SitesDev/wordpress/wp-admin/includes/template.php:1877) in /Users/dp/Documents/SitesDev/wordpress/wp-content/plugins/my-plugin/lib/Zebra/Zebra_Form.php on line 4052

Fatal error: The library tried to store the CSRF token in a cookie but was unable to do so because there was output already sent to the browser. You should either start a session prior to instantiating the library (recommended), have no output (including and tags, as well as any whitespace) sent to the browser prior to instantiating the library, or turn output buffering on in php.ini. in /Users/dp/Documents/SitesDev/wordpress/wp-content/plugins/my-plugin/lib/Zebra/Zebra_Form.php on line 4052

有什么线索吗？还是Zebra_form不能这样使用？