I checked our code and I believe we have the necessary null checks in the mentioned UpdateTabStopOnSelectionFromUI method.
However, looking at the last line in the stacktrace : System.Windows.Controls.ItemContainerGenerator.ContainerFromIndex(Int32 index,Boolean translateForItemsControl)
it seems that this might be also an issue in the Silverlight framework. Any additional information about this scenario might help us better investigate it.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.