title: Value from query string not seen in twig output
body: <p>I have a working Drupal 8 site with the following in THEME.theme:</p>
  
  <pre><code>function THEME_preprocess_node(&amp;$variables) {
    $vars['get']['vguid'] = \Drupal\Component\Utility\XSS::filter($_GET['vguid']);
  }
  </code></pre>
  
  <p>If I add the same line to THEME_preprocess_page() I get "Undefined index: in 'THEME_preprocess_page' when I attempt page loads.</p>
  
  <p>In the the twig I have <code>{{ get.vguid }}</code>  and have also tried <code>{{ app.request.parameter.get("vguid") }}</code></p>
  
  <p>In the twigs (after cache is cleared) I get no value when page is accessed like <code>/mobile-video?vguid=15991b1f-2ad2-11e7-8da8-22000aeb1f8b</code></p>
  
  <p>How do I get a sanitized query string value into my twig?</p>