Как исправить ошибку "в hrms возникло исключение типа 'system .nullreferenceexception'.dll, но не обрабатывается в пользовательском коде

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мой код выглядит примерно так

 [HttpPost]
        public FileResult DownloadFile(int? fileId)
        {
            byte[] bytes;
            string fileName, contentType;
            string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand())
                {
                    cmd.CommandText = "SELECT Name, Data, ContentType FROM tblFiles WHERE Id=@Id";
                    cmd.Parameters.AddWithValue("@Id", fileId);
                    cmd.Connection = con;
                    con.Open();
                    using (SqlDataReader sdr = cmd.ExecuteReader())
                    {
                        sdr.Read();
                        bytes = (byte[])sdr["Data"];
                        contentType = sdr["ContentType"].ToString();
                        fileName = sdr["Name"].ToString();
                    }
                    con.Close();
                }
            }

            return File(bytes, contentType, fileName);
        }

        private static List<FileModel> GetFiles()
        {
            List<FileModel> files = new List<FileModel>();
            string constr = ConfigurationManager.ConnectionStrings["Constring"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand("SELECT Id, Name FROM tblFiles"))
                {
                    cmd.Connection = con;
                    con.Open();
                    using (SqlDataReader sdr = cmd.ExecuteReader())
                    {
                        while (sdr.Read())
                        {
                            files.Add(new FileModel
                            {
                                Id = Convert.ToInt32(sdr["Id"]),
                                Name = sdr["Name"].ToString()
                            });
                        }
                    }
                    con.Close();
                }
            }
            return files;
        }
    }
}


The error is on 
string constr=ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;

What I have tried:

<pre>my code looks like this

<pre> [HttpPost]
        public FileResult DownloadFile(int? fileId)
        {
            byte[] bytes;
            string fileName, contentType;
            string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand())
                {
                    cmd.CommandText = "SELECT Name, Data, ContentType FROM tblFiles WHERE Id=@Id";
                    cmd.Parameters.AddWithValue("@Id", fileId);
                    cmd.Connection = con;
                    con.Open();
                    using (SqlDataReader sdr = cmd.ExecuteReader())
                    {
                        sdr.Read();
                        bytes = (byte[])sdr["Data"];
                        contentType = sdr["ContentType"].ToString();
                        fileName = sdr["Name"].ToString();
                    }
                    con.Close();
                }
            }

            return File(bytes, contentType, fileName);
        }

        private static List<FileModel> GetFiles()
        {
            List<FileModel> files = new List<FileModel>();
            string constr = ConfigurationManager.ConnectionStrings["Constring"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand("SELECT Id, Name FROM tblFiles"))
                {
                    cmd.Connection = con;
                    con.Open();
                    using (SqlDataReader sdr = cmd.ExecuteReader())
                    {
                        while (sdr.Read())
                        {
                            files.Add(new FileModel
                            {
                                Id = Convert.ToInt32(sdr["Id"]),
                                Name = sdr["Name"].ToString()
                            });
                        }
                    }
                    con.Close();
                }
            }
            return files;
        }
    }
}


The error is on 
string constr=ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;

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