Sometimes when Tab is pressed I get this exception:
An unhandled exception occured in the application
Message: Object reference not set to an instance of an object.
Exception type: System.NullReferenceException
Details:
   at Telerik.Windows.Controls.Input.FocusScope.FocusInternal() in c:\TB\117\WPF_Scrum\Release_WPF\Sources\Development\Core\Controls\Input\FocusScope.cs:line 146
   at Telerik.Windows.Controls.Input.FocusScope.FocusNextElement() in c:\TB\117\WPF_Scrum\Release_WPF\Sources\Development\Core\Controls\Input\FocusScope.cs:line 54
   at Telerik.Windows.Controls.Input.FocusScope.RemoveFocusableElement(Control element) in c:\TB\117\WPF_Scrum\Release_WPF\Sources\Development\Core\Controls\Input\FocusScope.cs:line 105
   at Telerik.Windows.Controls.Input.FocusScope.FocusableElement_Unloaded(Object sender, RoutedEventArgs e) in c:\TB\117\WPF_Scrum\Release_WPF\Sources\Development\Core\Controls\Input\FocusScope.cs:line 314
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   at MS.Internal.FrameworkObject.OnUnloaded(RoutedEventArgs args)
   at System.Windows.BroadcastEventHelper.BroadcastEvent(DependencyObject root, RoutedEvent routedEvent)
   at System.Windows.BroadcastEventHelper.BroadcastUnloadedEvent(Object root)
   at System.Windows.Media.MediaContext.FireLoadedPendingCallbacks()
   at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
   at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
   at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)