Can anyone tell me where I am going wrong in this code? I cant figure out the null reference.
All my code is below. I am exhausting myself. This is for a school project.
I have tried several different things, but none have seemed to work. Any suggestions would be appreciated.It worked, but when I rebooted it today it no longer worked.

enter image description here

using Microsoft.AspNetCore.Mvc;
using WeddingInvites.Data;
using WeddingInvites.Models;


namespace WeddingInvites.Controllers
{
    public class ResponseController : Controller
    {
        private GuestContext data { get; set; }

        public ResponseController(GuestContext ctx) => data = ctx;

        [HttpGet]
        public ViewResult ResponseForm()
        {
            return View();
        }

        [HttpPost]
        public IActionResult ResponseForm(Guest guest)
        {
            Guest check = data.Guests.FirstOrDefault(g => g.Name == guest.Name)!;
            if (check != null)
            {
                ModelState.AddModelError("Name", $"{guest.Name} has already submitted their RSVP.");
            }
            if (ModelState.IsValid)
            {
                data.Guests.Add(guest);
                data.SaveChanges();

                return View("SubmissionThanks");
            }
            else
            {
                ModelState.AddModelError("", "Please correct the errors.");
                return View();
            }
        }
        public ViewResult ListResponses()
        {
            return View();
        }
    }
}

@model Guest
@{
    ViewData["Title"] = "Wedding RSVP";
}
    <div class="text-center">
        <h2>Thanks for letting us know, @Model.Name</h2>
       <h4> @if(Model.WillAttend == true)
        {
            @:We are excited that you are able to celebrate our big day with us! Let the countdown for Mr. and Mrs. begin!
        }
        else
        {
            @:Ah shucks, we will miss you! It stinks that you can't attend, but thanks for your best wishes and for letting us know!
        }
        </h4>
    </div>
    <div class="text-center">Click <a asp-action="ListResponses" class="btn btn-sm btn-dark">here</a> to see the RSVP responses or close the browser if you are done.</div>

using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore.Storage.ValueConversion.Internal;
using System.ComponentModel.DataAnnotations;

namespace WeddingInvites.Models
{
    public class Guest
    {
        public int Id { get; set; }
        [Required(ErrorMessage = "Please enter your name.")]
        public string? Name { get; set; }
        [Required(ErrorMessage = "Please enter your email address.")]
        [RegularExpression(".+\@.+\..+", ErrorMessage = "Please enter a valid email address.")]
        public string? Email { get; set; }
        [Required(ErrorMessage = "Please enter your phone number.")]
        public string? Phone { get; set; }
        [Required(ErrorMessage = "Please specify whether you'll be in attendance or not.")]
        public bool? WillAttend { get; set; }
    }
}

@model Guest
@{
    string title = "Wedding RSVP Submission Form";
    ViewData["Title"] = title;
}
@section scripts {
    <script src="~/lib/jquery-validation/dist/jquery.validate.min.js"></script>
    <script src="~/lib/jquery-validation-unobtrusive/jquery.validate.unobtrusive.min.js"></script>
}

<h3>Please complete the form below:</h3>
    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
    <form asp-action="ResponseForm" method="post">
        <div class="mb-3">
            <label asp-for="Name" class="form-label">Your Name:</label>
            <span asp-validation-for="Name" class="text-danger"></span>
            <input asp-for="Name" class="form-control"/>
        </div>
        <div class="mb-3">
            <label asp-for="Email" class="form-label">Your Email:</label>
            <span asp-validation-for="Email" class="text-danger"></span>
            <input asp-for="Email" class="form-control" />
        </div>
        <div class="mb-3">
            <label asp-for="Phone" class="form-label">Your Phone Number:</label>
            <span asp-validation-for="Phone" class="text-danger"></span>
            <input asp-for="Phone" class="form-control" />

        </div>
        <div class="mb-3">
            <label asp-for="WillAttend" class="form-label">Will you be in attendance?</label>
            <span asp-validation-for="WillAttend" class="text-danger"></span>
            <select asp-for="WillAttend" class="form-select">
                <option value="">Choose an option</option>
                <option value="true">Yes, Count me in!</option>
                <option value="false">I can't make it, but I send my sincerest congratulations to y'all!</option>
            </select>
        </div>
        <button type="submit" class="btn btn-dark">Submit</button>
    <a asp-controller="Home" asp-action="Index" class="btn btn-dark">Back to Greeting</a>
</form>

@model List<Guest>
@{
    ViewData["Title"] = "Wedding RSVP";
}
    <h2 class="text-center text-warning">The list of Attendees</h2>
    <table class="table table-dark table-hover">
        <thead>
            <tr>
            <th>Name</th>
            <th>Email</th>
            <th>Phone Number</th>
            </tr>
        </thead>
            @foreach (Guest guest in Model)
            {
                <tr>
                    <td>@guest.Name</td>
                    <td>@guest.Email</td>
                    <td>@guest.Phone</td>
                </tr>
            }
    </table>
<a asp-controller="Home" asp-action="Index" class="btn btn-dark">Back to Greetings</a>

using Microsoft.EntityFrameworkCore;
using WeddingInvites.Models;

namespace WeddingInvites.Data
{
    public class GuestContext:DbContext
    {
        public GuestContext(DbContextOptions<GuestContext> options) : base(options) 
        { }
        public DbSet<Guest> Guests { get; set; } = null!;
    }
}