定时跳转页面报错,求答案,附源代码和错误WP8

Question

   

  错误信息如下,出现频率很高:

Navigation is not allowed when the task is not in the foreground.
   at System.Windows.Navigation.NavigationService.Navigate(Uri source)
   at carquey.StartMain.dTimer_Tick(Object sender, EventArgs e)
   at MS.Internal.CoreInvokeHandler.InvokeEventHandler(Int32 typeIndex, Delegate handlerDelegate, Object sender, Object args)
   at MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, Int32 actualArgsTypeIndex, String eventName)

   public partial class StartMain : PhoneApplicationPage
    {
        DispatcherTimer dRanTimer;
        int CountdownCount = 5;
        public StartMain()
        {
            InitializeComponent();          
            Countdown();
        }
        private void Countdown()
        {
            dRanTimer = new System.Windows.Threading.DispatcherTimer();
            dRanTimer.Interval = new TimeSpan(0, 0, 1);
            //设置事件处理函数
            dRanTimer.Tick += new EventHandler(dTimer_Tick);
            dRanTimer.Start();
        }
        private void dTimer_Tick(object sender, EventArgs e)
        {
            CountdownCount--;
            if (CountdownCount <= 0)
            {
                 dRanTimer.Stop();

// 这里出错误了啊?

              NavigationService.Navigate(new Uri("/MainPage.xaml", UriKind.RelativeOrAbsolute));
            }
        }
}

我将:

NavigationService.Navigate(new Uri("/MainPage.xaml", UriKind.RelativeOrAbsolute));

换成

 this.Dispatcher.BeginInvoke(() =>
                    {

                        NavigationService.Navigate(new Uri("/MainPage.xaml", UriKind.RelativeOrAbsolute));
                    });

问题依然存在.此错误只在部分机型,不是所有机型.