Object reference error when getting data from database - asp.net-mvc

Question

My portal has worked fine for several years but nowadays I have problems with my some codes. I want to update my code but at the first step I have problems with login page. when I use FirstOrDefault I got error on object reference while the list is filled and contains 1 user.
int g = db.sp_GetUserData_ByUserName(model.UserName.Trim()).Count();
//it shows 1
OSUserData userData1 = db.sp_GetUserData_ByUserName(model.UserName.Trim()).FirstOrDefault();
I didn't change this code and It works perfectly on the portal. However, I can't run it on my development server.
I checked again and I found that OSUserData constructor throws exception : Enumeration yielded no result. this is my class code :
public OSUserData()
{
this.OSUploadedFilesMultipleUsers = new HashSet<OSUploadedFilesMultipleUser>();
this.OSSalaryLists = new HashSet<OSSalaryList>();
this.OSUploadedFiles = new HashSet<OSUploadedFile>();
}
public int Id { get; set; }
public string Address { get; set; }
public string BranchCode { get; set; }
public string BranchParentNationalCode { get; set; }
public string CitName { get; set; }
public string City { get; set; }
public string CompanyName { get; set; }
public string CouName { get; set; }
public string Country { get; set; }
public string FaxNo { get; set; }
public string FollowCode { get; set; }
public string Hozeh { get; set; }
public Nullable<bool> IsBranch { get; set; }
public string LoginStatus { get; set; }
public string NationalCode { get; set; }
public string NewEconomicNo { get; set; }
public string Oldeconomicno { get; set; }
public string PostalCode { get; set; }
public string ProIdTTMS { get; set; }
public string ProName { get; set; }
public string Provision { get; set; }
public string RegistrationNo { get; set; }
public string State { get; set; }
public string TaxOfficeCode { get; set; }
public string TaxPayerType { get; set; }
public string TelCode { get; set; }
public string TelNo { get; set; }
public string TprId { get; set; }
public string UserName { get; set; }
public bool Validate { get; set; }
public bool Disabled { get; set; }
public string OwnerShipTypeDesc { get; set; }
public Nullable<byte> OwnerShipTypeCode { get; set; }
public bool Processed { get; set; }
public bool CheckedUser { get; set; }
public bool Blocked { get; set; }
public virtual ICollection<OSUploadedFilesMultipleUser> OSUploadedFilesMultipleUsers { get; set; }
public virtual ICollection<OSSalaryList> OSSalaryLists { get; set; }
public virtual ICollection<OSUploadedFile> OSUploadedFiles { get; set; }

Answer 1

You are trying to add to a null/unreferenced/not initialized object/list. Initialize first the list
List<OSUserData> userData1 = new List<OSUserData>();
Then add it
var user = db.sp_GetUserData_ByUserName(model.UserName.Trim()).FirstOrDefault();
userData1.Add(new OSUserData{
Prop1 = user.Prop1,
Prop2 = user.Prop2,
......
});
Or if you only want one user, just make it var
var user = db.sp_GetUserData_ByUserName(model.UserName.Trim()).FirstOrDefault();