Error generated from XML parsing code C#

System.InvalidOperationException was unhandled
  _HResult=-2146233079
  _message=Sequence contains no elements
  HResult=-2146233079
  IsTransient=false
  Message=Sequence contains no elements
  Source=System.Core
  StackTrace:
       at System.Linq.Enumerable.Last[TSource](IEnumerable`1 source)
       at RMBoink.MainWindow.Window_Loaded(Object sender, RoutedEventArgs e) in c:\Users\user\Documents\Visual Studio 2013\Projects\RMBoink\RMBoink\RMBoink\MainWindow.xaml.cs:line 74
       at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
       at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
       at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
       at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
       at System.Windows.BroadcastEventHelper.BroadcastEvent(DependencyObject root, RoutedEvent routedEvent)
       at System.Windows.BroadcastEventHelper.BroadcastLoadedEvent(Object root)
       at MS.Internal.LoadedOrUnloadedOperation.DoWork()
       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.Media.MediaContext.Resize(ICompositionTarget resizedCompositionTarget)
       at System.Windows.Interop.HwndTarget.OnResize()
       at System.Windows.Interop.HwndTarget.HandleMessage(WindowMessage msg, IntPtr wparam, IntPtr lparam)
       at System.Windows.Interop.HwndSource.HwndTargetFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
       at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
       at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
  InnerException: