HI,

 I was able to prevent the nullrefrence execption with the following code snippet:
 

 void expandBlock_MouseLeftButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e)  
       {       
      var r = (Row)_cell.Row;
            r.IsExpanded = !r.IsExpanded;
            ExpandColumnClickEventArgs _clickeventargs =   new ExpandColumnClickEventArgs(_cell.Row.Index, _cell.Row.Data);   
          if (_column != null)
            {
                _column.OnClick(_clickeventargs);       
      }
            else
            {                 _column = _cell.Column as ExpandColumn;  
                              _column.OnClick(_clickeventargs);
            }   
      }