How can I correctly run a timed process in my app the will start on stop when the application is in use or in the background?

I have an application that starts up and runs a background check of the database every minute. Below is the code for this.

I'm getting what I think is a memory leak and am looking at all areas of the code that loop.

Is there any possibility that this code could be left in a looping state and contribute to a memory leak or is the way the onSleep and onResume coded a 100% sure way to correctly stop and start the timer loop?

Note that I only want the timed part of the code to run once a minute when the application is being used and in the foreground.

namespace Japanese
{
    public partial class App : Application
    {

        private static Stopwatch stopWatch = new Stopwatch();

        public App()
        {
            InitializeComponent();
            MainPage = new Japanese.MainPage();
        }

        protected override void OnStart()
        {
            App.DB.InitData();

            if (!stopWatch.IsRunning) 
                stopWatch.Start();

            Device.StartTimer(new TimeSpan(0, 0, 1), () =>
            {
                if (stopWatch.IsRunning && stopWatch.Elapsed.Minutes >= defaultTimespan)
                {
                    Debug.WriteLine("Checking database");
                    PointChecker.CheckScore();
                    stopWatch.Restart();
                }
                return true;
            });
        }

        protected override void OnSleep()
        {
            stopWatch.Reset();
        }

        protected override void OnResume()
        {
            stopWatch.Start();
        }

    }
}