Xamarin Forms picker SelectedIndexChange null exception

Question

I have created an application that asks for a username and returns the stats for this user. When a counter is more than 1 it focuses on a picker and asks to choose one of the options. After, the SelectedIndexChanged (Platformselect_SelectedIndexChanged) method is being called and gets the stats. The problem is that if the user enters a username, the first time it works. When the user goes back and re-enters the username it doesn't focus (it doesn't show the picker) and automatically calls the SelectedIndexChanged method with platformselect.SelectedIndex being -1 causing my program to crash. Could you please help me?

I have tried to set focus on the main thread, I have tested it in both iOS and Android and the same problem appears.

public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
            platformselect.HeightRequest = 0;
            this.search.Clicked += Button_Clicked;
        }



        public async void searchStats(string usernameText)
        {
            search.IsEnabled = false;
            activityIndicator.IsRunning = true;
            var id = await GetDataLogic.GetID(usernameText);
            publicid = id;
            activityIndicator.IsRunning = false;

            if (id.uid == "error")
            {
                await DisplayAlert("Error", "Could not find username: " + usernameText, "OK");
                search.IsEnabled = true;
                return;
            }
            else if (id.uid == "errornointernet")
            {
                await DisplayAlert("Error", "Could not connect to the Internet. Please check your connection and try again.", "OK");
                search.IsEnabled = true;
                return;
            }
            else
            {
                if(id.platforms.Count > 1)
                {
                    platformselect.ItemsSource = publicid.platforms.ToList();
                    //we have to show alert to choose platform and then call get user stats.
                    **Device.BeginInvokeOnMainThread(() =>
                    {
                        if (platformselect.IsFocused)
                            platformselect.Unfocus();
                        platformselect.Focus();
                    });**
                }
                else
                {
                    //we call it without asking because there is only one platform.
                    var userstats = await GetDataLogic.GetStats(id, 0);
                    await Navigation.PushAsync(new Stats(userstats, favorites));
                }

            }

        }
        private async void Platformselect_SelectedIndexChanged(object sender, EventArgs e)
        {
            //the second time it doesnt show the picker and it automatically enters -1... it returns null exception. Has to be investigated.
            var userstats = await GetDataLogic.GetStats(publicid, platformselect.SelectedIndex);
            await Navigation.PushAsync(new Stats(userstats, favorites));
        }
    }

I would like it to always focus on the picker and wait for the response for the user, not automatically call SelectedIndexChanged with -1 as the selectedindex

Answer 1

I suggest change your event handler this way

 private async void Platformselect_SelectedIndexChanged(object sender, EventArgs e)
 {
    //Check for unselection event
    if(platformselect.SelectedIndex == -1) return;


    var userstats = await GetDataLogic.GetStats(publicid, platformselect.SelectedIndex);
    await Navigation.PushAsync(new Stats(userstats, favorites));
 }