User error: "1" is an invalid render array key in Drupal\Core\Render\Element::children()
Remove the brackets after the ['your_variable'] part as your variable holds only 1 array:

function MYTHEME_preprocess_views_view_fields(&amp;$vars) {
   $tempstore = \Drupal::service('user.private_tempstore')-&gt;get('evenementrimouski');
   $some_data = $tempstore-&gt;get('nid');
   $vars['your_variable'] = [3,4,5,6];
}


Also use the short array syntax. It's part of the new coding standards: https://www.drupal.org/node/2135291