Notice: Undefined index: plugin_id in custom_search_theme_suggestions_item_list__search_results() (line 109 of modules/contrib/custom_search/custom_search.pages.inc).

On a Drupal 8 site with Bootstrap theme I have exactly the same issue as described on https://www.drupal.org/project/custom_search/issues/2915553#comment-1232...

I gave it a few tries and, so far, I could not find any issues with bootstrap, but with the way custom_search module handles templating for the result set and result item. Also, on a cache clear, I see this: Notice: Undefined index: plugin_id in custom_search_theme_suggestions_item_list__search_results() (line 108 of modules/contrib/custom_search/custom_search.pages.inc).

Because the $variables['plugin_id'] in the following function sometimes is not defined:

/**
 * Implements hook_theme_suggestions_HOOK().
 */
function custom_search_theme_suggestions_item_list__search_results(array $variables) {
  return array('item_list__custom_search_results__' . $variables['plugin_id']);

}

So I am not sure what is the right solution, but putting the return array in an if condition takes care of the error:

/**
 * Implements hook_theme_suggestions_HOOK().
 */
function custom_search_theme_suggestions_item_list__search_results(array $variables) {
  if (isset($variables['plugin_id'])) {
    return array('item_list__custom_search_results__' . $variables['plugin_id']);
  }
}