Hello. I am facing an issue with null exception after calling OnPost method in my razor pages site.

 public IActionResult OnPostCalculateMortgage(int? id)
 {
     if (!ModelState.IsValid)
     {
         return Page();
     }

     ICalculateStrategy strategy;
     if (AmountToBorrow > MaximumLoan * 0.5m)
     {
         strategy = new FixedRateStrategy();
     }
     else
     {
         strategy = new FloatingRateStrategy();
     }

     Property.Calculator.SetStrategy(strategy);

     try
     {
         List<string> paymentSchedule = Property.Calculator.CalculateMonthlyPayment(AmountToBorrow, LoanTerm);

         ViewData["PaymentSchedule"] = paymentSchedule;
     }
     catch (InvalidOperationException ex)
     {
         ModelState.AddModelError(string.Empty, "An error occurred while calculating the mortgage.");
     }
     catch (NegativeLoanTermException ex)
     {
         ModelState.AddModelError(string.Empty, "Loan term must be positive and at least 12 months.");
     }
     catch (NegativePrincipalException ex)
     {
         ModelState.AddModelError(string.Empty, "The amount to borrow must be greater than zero.");
     }
     OnGet(id);
     return Page();
 }

 public IActionResult OnPostCalculateMortgage(int? id)
 {
     if (!ModelState.IsValid)
     {
         return Page();
     }

     ICalculateStrategy strategy;
     if (AmountToBorrow > MaximumLoan * 0.5m)
     {
         strategy = new FixedRateStrategy();
     }
     else
     {
         strategy = new FloatingRateStrategy();
     }

     Property.Calculator.SetStrategy(strategy);

     try
     {
         List<string> paymentSchedule = Property.Calculator.CalculateMonthlyPayment(AmountToBorrow, LoanTerm);

         ViewData["PaymentSchedule"] = paymentSchedule;
     }
     catch (InvalidOperationException ex)
     {
         ModelState.AddModelError(string.Empty, "An error occurred while calculating the mortgage.");
     }
     catch (NegativeLoanTermException ex)
     {
         ModelState.AddModelError(string.Empty, "Loan term must be positive and at least 12 months.");
     }
     catch (NegativePrincipalException ex)
     {
         ModelState.AddModelError(string.Empty, "The amount to borrow must be greater than zero.");
     }
     OnGet(id);
     return Page();
 }

 public Property GetPropertyDetails(int? id)

 public Property GetPropertyDetails(int? id)

            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                connection.Open();

                string query = "SELECT * FROM Properties WHERE PropertyID = @id";

                using (SqlCommand command = new SqlCommand(query, connection))
                {
                    command.Parameters.Add(new SqlParameter("@id", id));

                    using (SqlDataReader reader = command.ExecuteReader())
                    {

                        while (reader.Read())
                        {
                            List<string> urlList = !string.IsNullOrEmpty(urls) ? urls.Split(';').ToList() : new List<string>();

                            return new Property(pId, name, location, price, type, description, urlList, lat, lng);
                        }
                    }
                }
                return null;
            }

            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                connection.Open();

                string query = "SELECT * FROM Properties WHERE PropertyID = @id";

                using (SqlCommand command = new SqlCommand(query, connection))
                {
                    command.Parameters.Add(new SqlParameter("@id", id));

                    using (SqlDataReader reader = command.ExecuteReader())
                    {

                        while (reader.Read())
                        {
                            List<string> urlList = !string.IsNullOrEmpty(urls) ? urls.Split(';').ToList() : new List<string>();

                            return new Property(pId, name, location, price, type, description, urlList, lat, lng);
                        }
                    }
                }
                return null;
            }