GPP Child Theme/ function.php/ Fatal error (user logout)

Hi David,

I'd like registered users to click a logout-link in the header menu and then get redirected to a success page.

I was wondering if you could take a look at the php-code-snippet that I have added to the function.php:

//Redirect after logout to logout page
add_action('check_admin_referer', 'logout_without_confirm', 10, 2);
function logout_without_confirm($action, $result)
{
    /**
     * Allow logout without confirmation
     */
    if ($action == "log-out" && !isset($_GET['_wpnonce'])) {
        $redirect_to = isset($_REQUEST['redirect_to']) ? $_REQUEST['redirect_to'] : 'https://webgefaehrte.de/mitglieder/abgemeldet';
        $location = str_replace('&', '&', wp_logout_url($redirect_to));
        header("Location: $location");
        die;
    }
}
?> 

Debugging comes back with the following error unfortunately:

<br />
<b>Fatal error</b>:  Uncaught Error: Call to undefined function add_filter() in [...][...]:10
Stack trace:
#0 {main}
  thrown in <b>[...][...]</b> on line <b>10</b><br />

How would you go about fixing this error?

Thanks,
Jan