Null reference exception While Navigating to PlayListItem

I am trying to play the selected media from PlayList,if the selected index is not zero as below:

if (playList.Items.Count == 0)
   {
      setPlayList();
          if (selectedIndex!= 0)
               { 
                  if(custMediaElement.Playlist!=null)
                  custMediaElement.GoToPlaylistItem(selectedIndex);
               }
    }

But I am getting a Null reference Exc开发者_StackOverflow社区eption while trying to go to PlayList Item with the help of selected index , as explained in top. This works fine if I don't use custMediaElement.GoToPlaylistItem(selectedIndex); but in that case the Media Player Always plays the 1st Item , No matter , Which ever Song I select from the List Box.

Below is few details from Stack Trace:

ExpressionMediaPlayer.MediaPlayer.DoOpenPlaylistItem(PlaylistItem playlistItem)
at ExpressionMediaPlayer.MediaPlayer.GoToPlaylistItem(Int32 playlistItemIndex)

Thanks,

Subhen

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After digging through Got the method which worked :

 custMediaElement.GoToPlaylistItemOnNextTick(currentPlayListItem);