public ItemPage()
        {
            InitializeComponent();

            _navigationHelper = new NavigationHelper(this);
            _navigationHelper.LoadState += NavigationHelper_LoadState;
        } 
        public MainPage()
        {
            InitializeComponent();

            DisplayInformation.AutoRotationPreferences = DisplayOrientations.Portrait;

            NavigationCacheMode = NavigationCacheMode.Required;

            _navigationHelper = new NavigationHelper(this);
            _navigationHelper.LoadState += NavigationHelper_LoadState;
        }