Model.get returns null in View

Question

I am building a school assignment where I demonstrate a form where the user can input information that is then displayed, and they can view existing entries. I have a form where the user inputs data to do with listing a cat for adoption and when they click submit, it should display.

I am encountering the following problem when I try getting the model Cat in my Index view:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Microsoft.AspNetCore.Mvc.Razor.RazorPage.Model.get returned null.

My Index view where the error is occuring:

@model Cat
@{
    ViewData["Title"] = "Index";
}

<h2>Cat</h2>

<table class="table table-sm table-bordered table-striped">
    <tr><th>name:</th><td>@Model.Name</td></tr>
    <tr><th>Age:</th><td>@Model.Age</td></tr>
    <tr><th>Colouring:</th><td>@Model.Colouring</td></tr>
    <tr><th>KidFriendly:</th><td>@Model.KidFriendly</td></tr>
    <tr><th>Personality:</th><td>@Model.Personality</td></tr>
</table>

My Cat Details model:

using System.ComponentModel.DataAnnotations;

namespace IS4439Assignment1.Models
{
    public class Cat
    {
        public int Id { get; set; }

        
        [Required]
        public string? Name { get; set; }

        //age must be between 1 and 20
        [Required]
        public int Age { get; set; }

        public string? Sex { get; set; }

        //user is required to provide colouring details
        [Required]
        public string? Colouring { get; set; }

        //user is required to answer if the cat is kid friendly or not
        [Required]
        public KidFriendly KidFriendly { get; set; }

        //user is required to provide information about cat's personality, string length 100, minimum length 5
        [StringLength(100, MinimumLength = 5)]
        [Required]
        public string? Personality { get; set; }
    }
    //enum for multiple choice question on form
    public enum KidFriendly
    {
        Yes,
        No
    }
}

My Home controller:

using IS4439Assignment1.Models;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Mvc;
using System.Diagnostics;

namespace IS4439Assignment1.Controllers
{
    public class HomeController : Controller
    {
        public IActionResult Create() => View();

        [HttpPost]
      
        public IActionResult Create(Cat model) => View("Index", model);

        private IRepository repository;
        public HomeController(IRepository repo)
        {
            repository = repo;
        }
        public IActionResult Index(Cat catDetails)
        {
            if (catDetails.Id < 0)
                ModelState.AddModelError(nameof(catDetails.Id), "Id cannot be less than 0");

            if (string.IsNullOrEmpty(catDetails.Name))
                ModelState.AddModelError(nameof(Cat.Name), "Please enter the cats name");

            if (catDetails.Age < 0)
                ModelState.AddModelError(nameof(catDetails.Age), "Age cannot be less than 0");

            if (string.IsNullOrEmpty(catDetails.Sex))
                ModelState.AddModelError(nameof(catDetails.Sex), "Please select the cats sex");
            //if (catDetails.Colouring.ToString() == "Select")

                if (string.IsNullOrEmpty(catDetails.Colouring))
                ModelState.AddModelError(nameof(catDetails.Colouring), "Please select the cats colouring");

            if (ModelState.IsValid)
                return View("Index", catDetails);
            else
                return View();
        }
    }
}

I tried to test each line of the Index view where I am getting the model and each line breaks when trying to get the model, with the same error message. I believe the problem could be originating from a nullable value being returned from a dropdown item for the Colouring variable, however I have stated this String variable as nullable in my Cat Details model using

public string? Colouring { get; set; }

This was previously working so I do not understand where the error could have occured from. I am also a beginnner at C# so have little experience.