Destroy a Task

Hi Friends, and Happy Christmas to all of you.

I have this problem.
I have an app that, on OnStart(), create a Task

Task.Run(async()=> startTimer (60000));

this startTimer() has this code...

    private async Task startTimer(int delay){
        await App.UpdateDateTime (); // DO SOME WORKS
        App.CancellationToken = new CancellationTokenSource ();
        TimerRunning (delay, App.CancellationToken.Token);
    }

    private async void TimerRunning(int delay, CancellationToken token)
    {
        Debug.WriteLine ("Timer attivato");
        while (!token.IsCancellationRequested)
        {
            try
            {
                await Task.Delay(delay , token); // WAIT...
            }
            catch (TaskCanceledException)
            {
            }

            try{

                if (!token.IsCancellationRequested) {
                    await App.UpdateDateTime (); // REDO SOME WORKS
                }
            }
            catch{
            }
        }
    }

Then, on OnSleep() I try to suspend the Timer

    protected override void OnSleep ()
    {
        if (App.CancellationToken != null) {
            App.CancellationToken.Cancel ();
            App.CancellationToken = null;
        }

    }

It seems to work.

I have only this problem.

If I destroy my app (in Android visualizing all running app and switching my app to right...) I see that my app pass by "OnSleep" (so the thread should be cancelled), but if I RESTART my device (ON/OFF button and select Reset), at the startup i see this message

Unfortunately, (my app name) has stopped.

I have tried to comment the line (in OnStart())

Task.Run(async()=> startTimer (60000));

and do all steps (run my app, destroy it, restart the device...) and the "Unfortunately" message does not appear, so I think it's a "Task" problem.

Can someone explain to me how can I solve this situation? There is some way to destroy all Tasks before the app is killed?

Thanks