空引用发生在UpdatePanel中，因为渲染时数据源不可用

Question

我在标记页面和后端方法“ LoadUpdatePanels”中具有以下代码片段，以获取XML数据。

问题在于，由于XML尚未就绪，将异步加载，因此呈现UpdatePanel时会引发空引用错误。

我试图隐藏/显示它不起作用的UpdatePanel。

private void LibraryAddBoxFilters()
    {
        //create list of songs that match checked boxes
        using (var db = new TalLocalDbEntities())
        {
            //get filtered box with min number of tracks
            var id = db.TableBoxNames.Where(a => a.Filter == true).Min(a => a.Tracks);

            //get all songs from filtered box with min number of tracks
            var myResult = from b in db.TableBoxNames //get boxes
                           join n in db.TableBoxSongs on b.ID equals n.BoxID  //connect box id to boxid
                           where b.ID == id //select filtered box
                           select n //retrieve TableBoxSong
                            ;

            //for each Filter box selected
            foreach (var p in db.TableBoxNames.Where(a => a.Filter == true))
            {
                if (p.ID != id)//if not first table

                    //filter boxsongs
                    myResult.Intersect(from b in db.TableBoxNames //get boxes
                                       where b.ID == p.ID //select filtered box
                                       join n in db.TableBoxSongs on b.ID equals n.BoxID  //connect box id to boxid                                           
                                       select n); //retrieve TableBoxSong
            }

            //convert boxsongs to songs and send to library
            prog.CtrlMusicLibrary.matchfilter_Songs = (from m in myResult
                                                       join s in db.TableSongs on m.SongID equals s.ID
                                                       select s).Distinct().ToList();
        }
    }