title: Wordpress session is empty
body: <p>I'm trying to understand why the wordpress session doesnt't work.</p>
  <p>When I write <code>print_r($_SESSION);</code> it returns me empty array.</p>
  <p>I also tried with <code>$wp_session = WP_Session::get_instance();</code>, but it returns me error -
  There was a critical error on your website.</p>
  <p>What is the right way to access the session in wordpress? Thanks!</p>