BUG: Can't add multiple items to ListView (ArgumentOutOfRangeException)

I have confirmed this with both v1.1.1 and v1.2.1

Repro

1. create new PCL project
2. update Xamarin.Forms
3. copy the below code into your App.cs

4. run

   public class App
   {
       public static Page GetMainPage()
       {
           return new NavigationPage(new RootPage());
       }
   }
   
    public class RootPage : ContentPage
    {
       private ListView _listView;
   
       public RootPage()
       {
           var vm = new RootPageViewModel();
           Content = BuildListView(vm);
       }
   
       private StackLayout BuildListView(RootPageViewModel viewModel)
       {
           var headerTemplate = new DataTemplate(typeof(ModuleMediaListHeaderTemplate));
           headerTemplate.CreateContent();
   
           var itemTemplate = new DataTemplate(typeof(ModuleMediaListItemTemplate));
           itemTemplate.CreateContent();
   
           _listView = new ListView
           {
               ItemsSource = viewModel.MediaSections,
               IsGroupingEnabled = true,
               GroupDisplayBinding = new Binding("SectionName"),
               HasUnevenRows = false,
               GroupHeaderTemplate = headerTemplate,
               ItemTemplate = itemTemplate
           };
   
           return new StackLayout { Children = { _listView } };
       }
   }
   
   public class RootPageViewModel
   {
       public IEnumerable MediaSections
       {
           get
           {
               var titles = new[] {"First", "Second", "Third", "Forth", "Fifth"};
   
               return titles.Select(section => new MediaListSection(section)
               {
                   new FooViewModel {Title = "Foo", Description = "description for foo"}, 
                   new FooViewModel {Title = "Bar", Description = "description for bar"}, 
                   new FooViewModel {Title = "Baz", Description = "description for baz"}, 
                   new FooViewModel {Title = "Fiz", Description = "description for fiz"}, 
                   new FooViewModel {Title = "Buz", Description = "description for buz"},
               }).ToList();
           }
       }
   }
   
   public class MediaListSection : ObservableCollection<FooViewModel>
   {
       public string SectionName { get; private set; }
   
       public MediaListSection(string sectionName)
       {
           SectionName = sectionName;
       }
   }
   
   public class FooViewModel
   {
       public string Title { get; set; }
       public string Description { get; set; }
   }
   
   public class ModuleMediaListItemTemplate : ViewCell
   {
       public ModuleMediaListItemTemplate()
       {
           var title = new Label { YAlign = TextAlignment.Center };
           title.SetBinding(Label.TextProperty, new Binding("Title", BindingMode.OneWay));
   
           var description = new Label { YAlign = TextAlignment.Center };
           description.SetBinding(Label.TextProperty, new Binding("Description", BindingMode.OneWay));
   
           View = new StackLayout
           {
               Orientation = StackOrientation.Horizontal,
               Padding = new Thickness(8),
               Children = { title, description } 
           };
       }
   }
   
   public class ModuleMediaListHeaderTemplate : ViewCell
   {
       public ModuleMediaListHeaderTemplate()
       {
           var title = new Label { TextColor = Color.White, YAlign = TextAlignment.Center };
           title.SetBinding(Label.TextProperty, new Binding("SectionName", BindingMode.OneWay));
   
           View = new StackLayout
           {
               Padding = new Thickness(8, 0),
               BackgroundColor = Color.FromHex("#999999"),
               Orientation = StackOrientation.Horizontal,
               Children = { title },
           };
       }
   }