Warning:  Undefined array key "fk_i_category_id" in \item_online_users\functions.php on line 17

if($item_id > 0) {
    $count = iou_item_count($item_id);

    if($count !== false) {
      if(iou_param('category') != '') {
        $cats = explode(',', iou_param('category'));
        $item = Item::newInstance()->findByPrimaryKey($item_id);
       
        if(!in_array($item['fk_i_category_id'], $cats)) {
          return false;
        }
      }The floating bar works without a problem
In line does not work

Error basically means that item does not exists.
You can update that condition on line into:
if(!isset($item['pk_i_id']) || !in_array($item['fk_i_category_id'], $cats)) {