NullReferenceException: 未将对象引用设置到对象的实例。 StackTrace: 在 System.Windows.Interop.HwndKeyboardInputProvider.System.Windows.Input.IKeyboardInputProvider.AcquireFocus(Boolean checkOnly) 在 System.Windows.Input.KeyboardDevice.TryChangeFocus(DependencyObject newFocus, IKeyboardInputProvider keyboardInputProvider, Boolean askOld, Boolean askNew, Boolean forceToNullIfFailed) 在 System.Windows.Input.KeyboardDevice.Focus(DependencyObject focus, Boolean askOld, Boolean askNew, Boolean forceToNullIfFailed) 在 System.Windows.Input.KeyboardDevice.Focus(IInputElement element) 在 System.Windows.Interop.HwndKeyboardInputProvider.OnSetFocus(IntPtr hwnd) 在 System.Windows.Interop.HwndKeyboardInputProvider.FilterMessage(IntPtr hwnd, WindowMessage message, IntPtr wParam, IntPtr lParam, Boolean& handled) 在 System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) 在 MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) 在 MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o) 在 System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs) 在 System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)