在Xamarin.Forms中订阅事件时，为什么只能在iOS上收到NullReferenceException？

Question

I have a page - Page A, that has a method that subscribes to an event on another page - Page B. I figured I could instantiate Page B in my code in Page A before having my method in Page A subscribe to the event in Page B, and then finally pushing Page B to the navigation stack.

Unfortunately, I keep getting a NullReferenceException on the line in which the method subscribes to the event when I test my application on iOS. The code runs perfectly fine when I deploy and test as an Android application, but I always get the NullReferenceException on iOS. What's causing this exception to be thrown, and how can I fix it? Why is it platform specific to iOS?

Code on Page A

var confirmationPage = new EmailConfirmationPage(username);
confirmationPage.EmailConfirmed += this.OnEmailConfirmed;
await this.Navigation.PushModalAsync(confirmationPage);

...

private void OnEmailConfirmed(object source, EventArgs args)
{
    this.LabelMessage.Text = "Email Confirmed!";
}

Code on Page B

using System;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace appFBLA2019
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class EmailConfirmationPage : ContentPage
    {
        private string username;
        private string email;

        public delegate void EmailConfirmedEventHandler(object source, EventArgs args);
        public event EmailConfirmedEventHandler EmailConfirmed;

        public EmailConfirmationPage(string username)
        {
            InitializeComponent();
            this.username = username;
            this.LabelTitle.Text = "Loading...";
            GetEmail();
        }

        private void GetEmail()
        {
            try
            {
                ServerConnector.QueryDB($"getEmail/{this.username}/-");
                this.email = ServerConnector.ReceiveFromDB();
                this.LabelTitle.Text = $"Enter the confirmation code sent to {this.email.Split('/')[1]}";
            }
            catch
            {
                this.LabelMessage.Text = "Connection Error: Please Try Again.";
            }
        }

        private async void ButtonConfirmEmail_Clicked(object sender, EventArgs e)
        {
            try
            {
                string userInputToken = this.EntryConfirmationCode.Text.Trim();
                ServerConnector.QueryDB($"confirmEmail/{this.username}/{userInputToken}/-");
                string returnData = ServerConnector.ReceiveFromDB();

                if (returnData == "true/-")
                {
                    OnEmailConfirmed();
                    await this.Navigation.PopModalAsync(true);
                }
                else
                {
                    this.LabelMessage.Text = "Email could not be confirmed. Please try your code again.";
                }
            }
            catch
            {
                this.LabelMessage.Text = "Connection Error: Please try again.";
            }
        }

        private void ButtonFixEmail_Clicked(object sender, EventArgs e)
        {
            string newEmail = this.EntryChangeEmail.Text;
            ServerConnector.QueryDB($"changeEmail/{this.username}/{newEmail}/-");
            string result = ServerConnector.ReceiveFromDB();

            if (result == "true/-")
            {
                this.LabelMessage.Text = $"Enter the confirmation code sent to {newEmail}";
            }
            else
            {
                this.LabelMessage.Text = $"Email could not be changed: {result.Split('/')[1]}";
            }
        }

        private async void ButtonClose_Clicked(object sender, EventArgs e)
        {
            await this.Navigation.PopModalAsync(true);
        }

        protected virtual void OnEmailConfirmed()
        {
            EmailConfirmed?.Invoke(this, EventArgs.Empty);
        }
    }
}

Call Stack before subscribing method to event:

0xC0 in appFBLA2019.CreateAccountPage.ButtonCreateAccount_Clicked at C:\\Users\\chung\\source\\repos\\appFBLA2019\\appFBLA2019\\appFBLA2019\\CreateAccountPage.xaml.cs:30,21 C#

Call stack after subscribing method to event:

0x1B8 in appFBLA2019.CreateAccountPage.ButtonCreateAccount_Clicked at C:\\Users\\chung\\source\\repos\\appFBLA2019\\appFBLA2019\\appFBLA2019\\CreateAccountPage.xaml.cs:39,13 C#

Answer 1

Upon further testing, I noticed that this issue occurs on both iOS and Android, but ONLY when running the application with Xamarin Live Player. I contacted Microsoft and they pointed out that Xamarin Live Player unfortunately has limitations. Deploying directly to a device causes no issues, and the code runs fine.