<CollectionView.GroupFooterTemplate>
            <DataTemplate x:DataType="local:Group">
                <Border Background="Green"
                        HeightRequest="20">
                    <Label Text="{Binding Total}" />
                </Border>
            </DataTemplate>
        </CollectionView.GroupFooterTemplate>

   at System.Collections.ObjectModel.Collection`1.System.Collections.IList.Insert(Int32 index, Object value)
   at Microsoft.Maui.Controls.Platform.ObservableItemTemplateCollection.AddToSource(NotifyCollectionChangedEventArgs args)
   at Microsoft.Maui.Controls.Platform.ObservableItemTemplateCollection.TemplateCollectionChanged(Object sender, NotifyCollectionChangedEventArgs args)
   at System.Collections.ObjectModel.ObservableCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
   at System.Collections.ObjectModel.Collection`1.System.Collections.IList.Add(Object value)
   at Microsoft.Maui.Controls.Platform.GroupTemplateContext..ctor(ItemTemplateContext headerItemTemplateContext, ItemTemplateContext footerItemTemplateContext, Object items)
   at Microsoft.Maui.Controls.Platform.GroupedItemTemplateCollection.CreateGroupTemplateContext(Object group)
   at Microsoft.Maui.Controls.Platform.GroupedItemTemplateCollection..ctor(IEnumerable itemsSource, DataTemplate itemTemplate, DataTemplate groupHeaderTemplate, DataTemplate groupFooterTemplate, BindableObject container, IMauiContext mauiContext)
   at Microsoft.Maui.Controls.Platform.TemplatedItemSourceFactory.CreateGrouped(IEnumerable itemsSource, DataTemplate itemTemplate, DataTemplate groupHeaderTemplate, DataTemplate groupFooterTemplate, BindableObject container, IMauiContext mauiContext)
   at Microsoft.Maui.Controls.Handlers.Items.GroupableItemsViewHandler`1.CreateCollectionViewSource()
   at Microsoft.Maui.Controls.Handlers.Items.ItemsViewHandler`1.UpdateItemsSource()
   at Microsoft.Maui.Controls.Handlers.Items.SelectableItemsViewHandler`1.UpdateItemsSource()
   at Microsoft.Maui.Controls.Handlers.Items.ItemsViewHandler`1.MapItemsSource(ItemsViewHandler`1 handler, ItemsView itemsView)
   at Microsoft.Maui.PropertyMapper`2.<>c__DisplayClass5_0.<Add>b__0(IElementHandler h, IElement v)
   at Microsoft.Maui.PropertyMapper.UpdatePropertyCore(String key, IElementHandler viewHandler, IElement virtualView)
   at Microsoft.Maui.PropertyMapper.UpdateProperties(IElementHandler viewHandler, IElement virtualView)
   at Microsoft.Maui.Handlers.ElementHandler.SetVirtualView(IElement view)
   at Microsoft.Maui.Handlers.ViewHandler`2.SetVirtualView(IView view)
   at Microsoft.Maui.Handlers.ViewHandler`2.SetVirtualView(IElement view)
   at Microsoft.Maui.Controls.Element.SetHandler(IElementHandler newHandler)
   at Microsoft.Maui.Controls.Element.set_Handler(IElementHandler value)
   at Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler(IElementHandler value)
   at Microsoft.Maui.Platform.ElementExtensions.ToHandler(IElement view, IMauiContext context)
   at Microsoft.Maui.Platform.ElementExtensions.ToPlatform(IElement view, IMauiContext context)
   at Microsoft.Maui.Handlers.ContentViewHandler.UpdateContent(IContentViewHandler handler)
   at Microsoft.Maui.Handlers.ContentViewHandler.MapContent(IContentViewHandler handler, IContentView page)
   at Microsoft.Maui.PropertyMapper`2.<>c__DisplayClass5_0.<Add>b__0(IElementHandler h, IElement v)
   at Microsoft.Maui.PropertyMapper.UpdatePropertyCore(String key, IElementHandler viewHandler, IElement virtualView)
   at Microsoft.Maui.PropertyMapper.UpdateProperties(IElementHandler viewHandler, IElement virtualView)
   at Microsoft.Maui.Handlers.ElementHandler.SetVirtualView(IElement view)
   at Microsoft.Maui.Handlers.ViewHandler`2.SetVirtualView(IView view)
   at Microsoft.Maui.Handlers.ContentViewHandler.SetVirtualView(IView view)
   at Microsoft.Maui.Handlers.ViewHandler`2.SetVirtualView(IElement view)
   at Microsoft.Maui.Controls.Element.SetHandler(IElementHandler newHandler)
   at Microsoft.Maui.Controls.Element.set_Handler(IElementHandler value)
   at Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler(IElementHandler value)
   at Microsoft.Maui.Platform.ElementExtensions.ToHandler(IElement view, IMauiContext context)
   at Microsoft.Maui.Platform.ElementExtensions.ToPlatform(IElement view, IMauiContext context)
   at Microsoft.Maui.Handlers.WindowHandler.MapContent(IWindowHandler handler, IWindow window)
   at Microsoft.Maui.PropertyMapper`2.<>c__DisplayClass5_0.<Add>b__0(IElementHandler h, IElement v)
   at Microsoft.Maui.PropertyMapper.UpdatePropertyCore(String key, IElementHandler viewHandler, IElement virtualView)
   at Microsoft.Maui.PropertyMapper.UpdateProperties(IElementHandler viewHandler, IElement virtualView)
   at Microsoft.Maui.Handlers.ElementHandler.SetVirtualView(IElement view)
   at Microsoft.Maui.Controls.Element.SetHandler(IElementHandler newHandler)
   at Microsoft.Maui.Controls.Element.set_Handler(IElementHandler value)
   at Microsoft.Maui.Platform.ElementExtensions.SetHandler(IWinRTObject nativeElement, IElement element, IMauiContext context)
   at Microsoft.Maui.Platform.ElementExtensions.SetWindowHandler(Window platformWindow, IWindow window, IMauiContext context)
   at Microsoft.Maui.Platform.ApplicationExtensions.CreatePlatformWindow(Application platformApplication, IApplication application, OpenWindowRequest args)
   at Microsoft.Maui.Platform.ApplicationExtensions.CreatePlatformWindow(Application platformApplication, IApplication application, LaunchActivatedEventArgs args)
   at Microsoft.Maui.MauiWinUIApplication.OnLaunched(LaunchActivatedEventArgs args)
   at Microsoft.UI.Xaml.Application.Microsoft.UI.Xaml.IApplicationOverrides.OnLaunched(LaunchActivatedEventArgs args)
   at ABI.Microsoft.UI.Xaml.IApplicationOverrides.Do_Abi_OnLaunched_0(IntPtr thisPtr, IntPtr args)