why is Navigation.PushModalAsync returns null

when class SettingsPage included only InitializeComponent();, the navigation method worked fine. But when I add some method (below) in class SettingsPage , now It`s returns null.

Bulid and rebuild returns no errors and exceptions.

    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }

        private async void Button_Clicked(object sender, EventArgs e)
        {
            await this.Navigation.PushModalAsync(new SettingsPage()).ConfigureAwait(false);  //Returns Null
    
        }
       
    } 

this is my SettingsPage class :

    public partial class SettingsPage : ContentPage
    {
       readonly INotificationManager notificationManager;
        DateTime _triggerTime;
        public SettingsPage()
        {
            InitializeComponent();


            Device.StartTimer(TimeSpan.FromSeconds(1), OnTimerTick);

            notificationManager = DependencyService.Get<INotificationManager>();
            notificationManager.NotificationReceived += (sender, eventArgs) =>
            {
                var evtData = (NotificationEventArgs)eventArgs;
                ShowNotification(evtData.Title, evtData.Message);
            };
        }

        bool OnTimerTick()
        {
            if (DateTime.Now >= _triggerTime)
            {
                _switch.IsToggled = false;
                DisplayAlert("Alert", " It`s time to drink water :) ", "OK");
                string title = $"Drink Water Reminder";
                string message = $" It`s time to drink water :) ";
                notificationManager.ScheduleNotification(title, message);



                if (_switchVibrate.IsToggled)
                {
                    // Use default vibration length
                    Vibration.Vibrate();

                    // Or use specified time
                    var duration = TimeSpan.FromSeconds(1);
                    Vibration.Vibrate(duration);
                }

            }
            return true;
        }

        void OnTimePickerPropertyChanged(object sender, PropertyChangedEventArgs args)
        {
            if (args.PropertyName == "Time")
            {
                SetTriggerTime();
            }
        }

        void OnSwitchToggled(object sender, ToggledEventArgs args)
        {
            SetTriggerTime();
        }

        void SetTriggerTime()
        {
            if (_switch.IsToggled)
            {
                _triggerTime = DateTime.Today + _timePicker.Time;
                if (_triggerTime < DateTime.Now)
                {
                    _triggerTime += TimeSpan.FromDays(1);
                }
            }
        }
        void ShowNotification(string title, string message)
        {
            Device.BeginInvokeOnMainThread(() =>
            {
                var msg = new Label()
                {
                    Text = $"Notification Received:\nTitle: {title}\nMessage: {message}"
                };

            });
        }

        private void Button_Clicked(object sender, EventArgs e)
        {
            OnTimerTick();

        }

    }