Error on hook_theme_suggestions_HOOK_alter()
Every time it calls the hook, the variables may or may not have the array key 'attributes' - so the error happens on certain hook call, but not others. It's easy to fix, just check if it exists

function MYTHEME_theme_suggestions_item_list_alter(array &amp;$suggestions, array $variables) {
  if (!empty($variables['attributes'])) {
    $moduleclass = str_replace('-', '_', $variables['attributes']['class']);
    $suggestions[] = 'item_list__' . $moduleclass;
  }
}