Why this object reference is not set to an instance of an object?

Question

I am building an invoice app that has basic CRUD functionality. Whenever I am trying to change the status of an invoice and redirect the user back to ViewInvoice page, I get a NullReferenceException for property Status of an Invoice object. What could be the reason for that ?

Here’s my Invoice model:

public class Invoice
{
    public string InvoiceID { get; set; } = GenerateID.GenerateInvoiceID();
    public string Description { get; set; }
    public List<InvoiceItem> Items { get; set; }
    public DateTime InvoiceDate { get; set; }
    public string PaymentTerms { get; set; }
    public DateTime PaymentDue { get; set; }
    public int TotalPrice { get; set; }
    public string Status { get; set; } = "pending";
    public Client Client { get; set; }
   
    
}

Here’s the reference to the status property and the form in the ViewInvoice View page that triggers the request by calling OnPostPaid handler in ViewInvoice PageModel:

<p class="status-value">@Model.Invoice.Status</p>

<form asp-page-handler="paid" method="post">
    <button class="mark-paid-link invoice-control-link">Mark as Paid</button>
</form>

Here’s ViewInvoice PageModel:

        public Invoice Invoice { get; set; }
    

    public readonly InvoiceContext _context;
    public readonly InvoiceService _service;

    public ViewInvoiceModel(InvoiceContext context, InvoiceService service)
    {
        _context = context;
        _service = service;
    }


    public IActionResult OnGet(string id)
    {
        
        Invoice = IndexModel.invoices.Find(invoice => invoice.InvoiceID == id);
        if(Invoice is null)
        {
            return NotFound();
        }

        return Page();
    }

    public IActionResult OnPostDelete(string id)
    {
        
        IndexModel.invoices.Remove(IndexModel.invoices.Find(item => item.InvoiceID == id));
        return RedirectToPage("/index");
    }

    public void OnPostPaid(string id)
    {
        Invoice invoice = IndexModel.invoices.Find(invoice => invoice.InvoiceID == id);
        if(invoice != null)
        {
            invoice.Status = "Paid";
        }
        
  
    }