"ApiKey" Parameter is null during register confirmation by Email

Question

I have a problem with register confirmation. It displays an ArgumentNullException and I coudln't solve this problem.

This is RegisterConfirmationModel

[AllowAnonymous]
public class RegisterConfirmationModel : PageModel
{
    private readonly UserManager<ScarletSunset.Website.User.Areas.Identity.Data.User> _userManager;
    private readonly IEmailSender _sender;

    public RegisterConfirmationModel(UserManager<ScarletSunset.Website.User.Areas.Identity.Data.User> userManager, IEmailSender sender)
    {
        _userManager = userManager;
        _sender = sender;
    }

    public string Email { get; set; }

    public bool DisplayConfirmAccountLink { get; set; }

    public string EmailConfirmationUrl { get; set; }

    public async Task<IActionResult> OnGetAsync(string email, string returnUrl = null)
    {
        if (email == null)
        {
            return RedirectToPage("/Index");
        }

        var user = await _userManager.FindByEmailAsync(email);
        if (user == null)
        {
            return NotFound($"Unable to load user with email '{email}'.");
        }

        Email = email;
        // Once you add a real email sender, you should remove this code that lets you confirm the account
        DisplayConfirmAccountLink = false;
        if (DisplayConfirmAccountLink)
        {
            var userId = await _userManager.GetUserIdAsync(user);
            var code = await _userManager.GenerateEmailConfirmationTokenAsync(user);
            code = WebEncoders.Base64UrlEncode(Encoding.UTF8.GetBytes(code));
            EmailConfirmationUrl = Url.Page(
                "/Account/ConfirmEmail",
                pageHandler: null,
                values: new { area = "Identity", userId = userId, code = code, returnUrl = returnUrl },
                protocol: Request.Scheme);
        }           

        return Page();
    }
}

This is EmailSender, I took it from Microsoft docs

public class EmailSender : IEmailSender
{
    public EmailSender(IOptions<AuthMessageSenderOptions> optionsAccessor)
    {
        Options = optionsAccessor.Value;
    }

    public AuthMessageSenderOptions Options { get; } //set only via Secret Manager

    public Task SendEmailAsync(string email, string subject, string message)
    {
        return Execute(Options.SendGridKey, subject, message, email);
    }

    public Task Execute(string apiKey, string subject, string message, string email)
    {
        var client = new SendGridClient(apiKey);
        var msg = new SendGridMessage()
        {
            From = new EmailAddress("[email protected]", Options.SendGridUser),
            Subject = subject,
            PlainTextContent = message,
            HtmlContent = message
        };
        msg.AddTo(new EmailAddress(email));

        // Disable click tracking.
        // See https://sendgrid.com/docs/User_Guide/Settings/tracking.html
        msg.SetClickTracking(false, false);

        return client.SendEmailAsync(msg);
    }
}

---

When I register a new User it shows this Error. Error is in the EmailSender class, Execute Method. And I don't understand why is ApiKey null. Please, help me to solve this.