title: Grid.Add.Children(UIElement) returns Parameter is incorrect exception
body: <p>I am trying to add border in two pivot items. </p>
  
  <p>When my border is added to grid in pivot item for the first time everything is working fine. But when i try to add border second time in same pivot item it throws an exception "The parameter is incorrect"
   here is my code : </p>
  
  <pre><code>private void pivot_item1Loaded()
          {
              WebClient webClient2011 = new WebClient();
              string Url2011 = "http://hostname/Details/Images?year=2011" + "&amp;time=" + System.DateTime.UtcNow;
              webClient2011.OpenReadAsync(new Uri(Url2011));
  
              webClient2011.OpenReadCompleted += new OpenReadCompletedEventHandler(webClient_OpenReadCompleted2011);
          }
  
          private void pivot_item2Loaded()
          {
              WebClient webClient2012 = new WebClient();
              string Url2012 = "http://hostname/Details/Images?year=2012" +"&amp;time="+ System.DateTime.UtcNow;
              webClient2012.OpenReadAsync(new Uri(Url2012));
  
              webClient2012.OpenReadCompleted += new OpenReadCompletedEventHandler(webClient_OpenReadCompleted2012);
          }
          public void webClient_OpenReadCompleted2011(object sender, OpenReadCompletedEventArgs e)
          {
              StringBuilder output = new StringBuilder();
              try
              {               
                  using (XmlReader reader = XmlReader.Create(e.Result))
                  {
  
                      while (reader.Read())
                      {
                          if (reader.NodeType == XmlNodeType.Element)
                          {
                              if (reader.Name == "iconPath")
                              {
                                  string iconPath = reader.ReadElementContentAsString();
                                  iconImages2011.Add(iconPath);
                              }
                              if (reader.Name == "imagePath")
                              {
                                  string imagePath = reader.ReadElementContentAsString();
                                  fullScreenImages2011.Add(imagePath);
                              }
                          }
  
                      }
                  }
  
                  int numOfRows = (iconImages2011.Count) / 3 + 1;
                  for (int j = 0; j &lt; numOfRows; j++)
                  {
                      //pivot_item1 
                      ContentPanel2011.RowDefinitions.Add(new RowDefinition() { Height = new GridLength(150) });
                  }
                  int rowCount = 0;
                  int columnCount = 0;
  
                  for (int i = 0; i &lt; iconImages2011.Count; i++)
                  {
                      Border border2011 = new Border();
                      border2011.Background = new SolidColorBrush(Colors.Blue);
  
                      border2011.Height = 110;
                      border2011.Width = 110;
                      border2011.CornerRadius = new CornerRadius(10);
  
                      Canvas canvas2011 = new Canvas();
                      canvas2011.Height = 110;
                      canvas2011.Width = 110;
  
                      BitmapImage AppImage = new BitmapImage(new Uri(iconImages2011[i], UriKind.Absolute));
  
                      Image img = new Image();
                      img.Source = AppImage;
                      img.Width = 90;
                      img.Height = 90;
                      img.Stretch = Stretch.Fill;
                      img.Margin = new Thickness(10, 10, 10, 10);
                      canvas2011.Children.Add(img);
                      border2011.Child = canvas2011;
                      border2011.Name = i.ToString();
  
                      Grid.SetColumn(border2011, columnCount);
                      Grid.SetRow(border2011, rowCount);
  
                      ContentPanel2011.Children.Add(border2011);
  
                      pivot2011.Content = ContentPanel2011;
                      if (columnCount &lt; 2)
                      {
                          columnCount++;
                      }
                      else if (columnCount == 2)
                      {
                          columnCount = 0;
                          rowCount++;
                      }
  
                  }
  
              }
  
              catch (Exception x)
              {
                  MessageBox.Show(x.Message);
              }
  
          }
  </code></pre>
  
  <p>This code works for the first time but gives exception after that and ContentPanel2011 viz pivot_item1 do not get filled with border2011</p>