Index '1' is greater than the number of rows '0'. (XF 240-Pre1 iOS)

Just updated to 240-pre1 and now I cannot add items to an observable collection bound to a ListView with Uneven Rows and Groups. Have only tried iOS so far.

ObservableCollection<Group> groupedItems = new ObservableCollection<Group>();
listView.ItemsSource = groupedItems;
                        Group group1 = new Group("Text", "Text2");
                        groupedItems.Add(group1);

                        Group group2 = new Group("Text", "Text2");
                        groupedItems.Add(group2);   ***** Crash here *********

The stack trace is emense. Here is the first portion.
" at Xamarin.Forms.Platform.iOS.ListViewRenderer.UpdateItems (System.Collections.Specialized.NotifyCollectionChangedEventArgs e, System.Int32 section, System.Boolean resetWhenGrouped) [0x0006b] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Platform.iOS\Renderers\ListViewRenderer.cs:509 \n at Xamarin.Forms.Platform.iOS.ListViewRenderer.OnCollectionChanged (System.Object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Platform.iOS\Renderers\ListViewRenderer.cs:300 \n at Xamarin.Forms.Internals.TemplatedItemsList2[TView,TItem].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x0000a] in C:\\BuildAgent3\\work\\ca3766cfc22354a1\\Xamarin.Forms.Core\\TemplatedItemsList.cs:758 \n at Xamarin.Forms.Internals.TemplatedItemsList2[TView,TItem].OnCollectionChangedGrouped (System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x000d5] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\TemplatedItemsList.cs:785 \n at Xamarin.Forms.Internals.TemplatedItemsList2[TView,TItem].OnProxyCollectionChanged (System.Object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e, System.Boolean fixWindows) [0x00008] in C:\\BuildAgent3\\work\\ca3766cfc22354a1\\Xamarin.Forms.Core\\TemplatedItemsList.cs:962 \n at Xamarin.Forms.Internals.TemplatedItemsList2[TView,TItem].OnProxyCollectionChanged (System.Object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\TemplatedItemsList.cs:955 \n at Xamarin.Forms.ListProxy.OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x0000a] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\ListProxy.cs:233 \n at Xamarin.Forms.ListProxy+<>c__DisplayClass33_0.b__0 () [0x00018] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\ListProxy.cs:206 \n at Xamarin.Forms.ListProxy.OnCollectionChanged (System.Object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x000a0] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\ListProxy.cs:225 \n at Xamarin.Forms.ListProxy+WeakNotifyProxy.OnCollectionChanged (System.Object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x00031] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\ListProxy.cs:394 \n at System.Collections.ObjectModel.ObservableCollection1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x0000f] in /Library/Frameworks/Xamarin.iOS.framework/Versions/10.12.0.18/src/mono/mcs/class/referencesource/System/compmod/system/collections/objectmodel/observablecollection.cs:288 \n at System.Collections.ObjectModel.ObservableCollection1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedAction action, System.Object item, System.Int32 index) [0x00000] in /Library/Frameworks/Xamarin.iOS.framework/Versions/10.12.0.18/src/mono/mcs/class/referencesource/System/compmod/system/collections/objectmodel/observablecollection.cs:351 \n

Am I doing something wrong? Thanks.