what were you doing when this occurred ?
Always when close the program.
Debug:System.NullReferenceException: Odwołanie do obiektu nie zostało ustawione na wystąpienie obiektu.
   w MixedInKey.Services.SystemEventsListener.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   w System.Windows.Interop.HwndSource.PublicHooksFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   w MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   w MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   w System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   w System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
or 
Event Log:
Aplikacja: MixedInKey.exe
Wersja architektury: v4.0.30319
Opis: 
aplikacja zażądała przerwania procesu za pomocą metody System.Environment.FailFast(ciąg komunikatu).
Komunikat: Fatal error: System.NullReferenceException: Odwołanie do obiektu nie zostało ustawione na wystąpienie obiektu.   w MixedInKey.Services.SystemEventsListener.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   w System.Windows.Interop.HwndSource.PublicHooksFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   w MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   w MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   w System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   w System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
Stack:
   w System.Environment.FailFast(System.String)
   w MixedInKey.App.PromptUserAboutUnhandledException(System.Exception)
   w MixedInKey.App.ReportUnhandledException(System.Exception)
   w MixedInKey.App.Current_DispatcherUnhandledException(System.Object, System.Windows.Threading.DispatcherUnhandledExceptionEventArgs)
   w System.Windows.Threading.Dispatcher.CatchException(System.Exception)
   w System.Windows.Threading.Dispatcher.CatchExceptionStatic(System.Object, System.Exception)
   w System.Windows.Threading.ExceptionWrapper.CatchException(System.Object, System.Exception, System.Delegate)
   w System.Windows.Threading.ExceptionWrapper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
   w System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority, System.TimeSpan, System.Delegate, System.Object, Int32)
   w MS.Win32.HwndSubclass.SubclassWndProc(IntPtr, Int32, IntPtr, IntPtr)
   w MS.Win32.UnsafeNativeMethods.IntDestroyWindow(System.Runtime.InteropServices.HandleRef)
   w MS.Win32.HwndWrapper.DestroyWindow(System.Object)
   w MS.Win32.HwndWrapper.Dispose(Boolean, Boolean)
   w MS.Win32.HwndWrapper.Dispose()
   w System.Windows.Interop.HwndSource.Dispose(Boolean)
   w System.Windows.Interop.HwndSource+WeakEventDispatcherShutdown.OnShutdownFinished(System.Object, System.EventArgs)
   w System.EventHandler.Invoke(System.Object, System.EventArgs)
   w System.Windows.Threading.Dispatcher.ShutdownImplInSecurityContext(System.Object)
   w System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   w System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   w System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
   w MS.Internal.CulturePreservingExecutionContext.Run(MS.Internal.CulturePreservingExecutionContext, System.Threading.ContextCallback, System.Object)
   w System.Windows.Threading.Dispatcher.ShutdownImpl()
   w System.Windows.Threading.Dispatcher.PushFrameImpl(System.Windows.Threading.DispatcherFrame)
   w System.Windows.Threading.Dispatcher.PushFrame(System.Windows.Threading.DispatcherFrame)
   w System.Windows.Application.RunDispatcher(System.Object)
   w System.Windows.Application.RunInternal(System.Windows.Window)
   w System.Windows.Application.Run(System.Windows.Window)
   w MixedInKey.App.Main()
