Insert data into database(MVC)

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MVC

data

I am trying to insert datas to Appointment table of my database.I did registeration part of my project which works well.there are 2 tables ,Patient and Appointment.After Login patients can make an appointment.Patient number comes like this

MyUser.PatientNo = Guid.NewGuid().GetHashCode();

For appointment date and description comes from textbox.And i want to insert PatientNo from Patient table to Appointment table.For me it looks done but when i choose date and write description but i got error on this line " app.PatientNo = patient.PatientNo;"

An exception of type 'System.NullReferenceException' occurred in DentAppSys.dll but was not handled in user code

[HttpPost]
           [ValidateAntiForgeryToken]
           public ActionResult Make(Models.AppModel User)
           {
               if (Session["UserEmail"] != null)
               {
                   using (var db = new MaindbModelDataContext())
                   {


                       var patient = db.Patients.FirstOrDefault(u => u.Email == (String)Session["UserEmail"]);
                       var app = new Appointment();
                       app.Date = User.Date;
                       app.Description = User.Description;
                       app.Status = "true";
                       app.PatientNo = patient.PatientNo;
                       db.Appointments.InsertOnSubmit(app);
                       db.SubmitChanges();
                       return RedirectToAction("Make", "Appointment");
                   }

               }
               else
               {
                   return RedirectToAction("Index", "User");
               }
           }

       }
   }

and this is registration part which is working well

public ActionResult RegAndLogin(Models.RegAndLog User)
            {
                if (User.RegisterModel != null)
                {
                    if (ModelState.IsValid)
                    {
                        using (var db = new MaindbModelDataContext())
                        {
                            var Person = db.Patients.FirstOrDefault(u => u.Email == User.RegisterModel.Email);
                            if (Person == null)
             {  string Hash = BCrypt.Net.BCrypt.HashPassword(User.RegisterModel.Password);
                                var MyUser = new Patient();
                                MyUser.Name = User.RegisterModel.Firstname;
                                MyUser.Surname = User.RegisterModel.Lastname;
                                MyUser.Birthday = User.RegisterModel.Birthday;
                                MyUser.Email = User.RegisterModel.Email;
                                MyUser.Password = Hash;
                                MyUser.PatientNo = Guid.NewGuid().GetHashCode();
                                db.Patients.InsertOnSubmit(MyUser);
                                db.SubmitChanges();
    
                                Session["UserEmail"] = User.RegisterModel.Email;
                                return RedirectToAction("Index", "Patient", User.RegisterModel);
                            }                else                      {
                                ModelState.AddModelError("", "There is a user with this Email. Please enter another Email !!!");
                                return View();
                            } }}
                    else  {  ModelState.AddModelError("", "Data is incorrect !!!");  }}
                else
        { if (ModelState.IsValid && IsValid(User.LoginModel.Email, User.LoginModel.Password))
                    {
                        var TempUser = new Models.RegisterModel();
                        Session["UserEmail"] = User.LoginModel.Email;
                        using (var db = new MaindbModelDataContext())
                        {
                            var person = db.Patients.FirstOrDefault(u => u.Email == User.LoginModel.Email);
                            TempUser.Firstname = person.Name;
                           TempUser.Lastname = person.Surname;
                            //TempUser.RegisterModel.Birthday = (DateTime)person.BirthDate;
                            TempUser.Email = person.Email;
    
    
                        }
                        return RedirectToAction("Index", "Patient", TempUser);
    
    
                    }
                    else
                    {
                        ModelState.AddModelError("", "Check your E-mail or Password then try again !!!");
                    }
                }
                return View();