Handling 'Sequence has no elements' exception dup - sql

Question

I just finished my ASP.NET hospital management app, everything works fine, but if I try to add a doctor I get this error:
System.InvalidOperationException HResult=0x80131509
Message=Sequence contains no elements
Source=Hospital Management System
StackTrace:
at Hospital_Management_System.Controllers.DoctorController.Index(String message) in C:\Users\mouha\OneDrive\Bureau\Hospital Management System\Hospital Management System\Controllers\DoctorController.cs:line 37
at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary2 parameters) at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary2 parameters)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.b__3d()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<>c__DisplayClass46.b__3f()
The code:
public ActionResult Index(string message)
{
var date = DateTime.Now.Date;
ViewBag.Messege = message;
var user = User.Identity.GetUserId();
// The exception is being thrown here
var doctor = db.Doctors.Single(c => c.ApplicationUserId == user);
var model = new CollectionOfAll
{
Ambulances = db.Ambulances.ToList(),
Departments = db.Department.ToList(),
Doctors = db.Doctors.ToList(),
Patients = db.Patients.ToList(),
Medicines = db.Medicines.ToList(),
ActiveAppointments = db.Appointments.Where(c => c.DoctorId == doctor.Id).Where(c => c.Status).Where(c => c.AppointmentDate >= date).ToList(),
PendingAppointments = db.Appointments.Where(c => c.DoctorId == doctor.Id).Where(c => c.Status == false).Where(c => c.AppointmentDate >= date).ToList(),
AmbulanceDrivers = db.AmbulanceDrivers.ToList(),
Announcements = db.Announcements.Where(c => c.AnnouncementFor == "Doctor").ToList()
};
return View(model);
}