Error

Hi support!
In my wordpress admin appear a code
Notice: Undefined index: custom_css_noncename in /var/www/html/wp-content/plugins/rt-custom-css-page-and-post/rt-custom-css-page-and-post.php on line 26

What can I do ?

That’s the code in line 26

if (!wp_verify_nonce($_POST[‘custom_css_noncename’], ‘custom-css’)) return $post_id;

Thank you

https://wordpress.org/plugins/rt-custom-css-page-and-post/