ASP.NET core razor page value cannot be null

I'm new to ASP.NET core and razor page. I hav ecreated a button and assigned a page handler to delete the entry. When I'm trying to delete im getting a null reference error. When I run the code and click the button to delete an entry, im getting a null exception error

I want to know where I'm making mistake and some guidance to improve my coding. Thank you in advance for those who helped.

This is my index page

@page
@model BookList.Pages.BookPages.IndexModel

       
            <form method="post">
                @if (Model.Books.Count()>0)
                {
                    <table class="table table-dark border">
                        <tr class="table-secondary">
                         @foreach (var book in Model.Books)
                        {
                            <tr>
                                <td>
                                    @Html.DisplayFor(m => book.Name)
                                </td>
                                <td>
                                    @Html.DisplayFor(m => book.Author)
                                </td>
                                 <td>
                                    <button class="btn btn-danger btn-sm" asp-page-handler="Delete" asp-route-id="@book.Id" onclick="return confirm('Are you sure you want to delete?')">Delete</button>
                                    <a class="btn btn-success btn-sm" asp-route-id="@book.Id" asp-page="Edit">Edit</a>
                                   
                                </td>
                            </tr>
                        }
                    </table>
                }
                else
                {
                    <p>No books available</p>
                }
            </form>
       

This is my page model

        public IEnumerable<Book> Books { get; set; }

        public async Task OnGet()
        {
            Books = await _dbContext.Books.ToListAsync();
        }
        
        
        public async Task<IActionResult> onPostDelete(int id)
        {
            var book = await _dbContext.Books.FindAsync(id);

            if (book == null)
                return NotFound();

            _dbContext.Books.Remove(book);

            await _dbContext.SaveChangesAsync();

            return RedirectToPage("Index");

        }
    }
}