Login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection.

using System;
using System.Collections.Generic;
using System.Text;
using System.Data.SqlClient;


namespace Faculty_Schedule_Time_Table
{

        public class Global_Function
    {

            public SqlConnection con = new SqlConnection();
            private string connectionstring ="Data Source=NARASIMAN;connect timeout=120;initial catalog=master";
            private SqlCommand cmd;
            private SqlDataReader dr;

            public string error;

            public Global_Function()
            {

            }

            public void BindCon()
            {
                con = new SqlConnection();
                con.ConnectionString = connectionstring;
                con.Open();

            }

            public void insertdata(String SQL)
            {
                try
                {
                    BindCon();
                    cmd = new SqlCommand(SQL, con);
                    cmd.ExecuteReader();
                }
                 catch(Exception e1)
                  {
                   error = e1.Message.ToString();
                  }
            }
            public SqlDataReader ReadSql(string SQL)
            {

            con = new SqlConnection(connectionstring);
            con.Open();
            cmd  = new SqlCommand(SQL,con);
            dr = cmd.ExecuteReader();
            return (dr);
            
        }
           
            public string CntString()
        {
            return connectionstring;
        }
    }
}

Login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection.

public SqlDataReader ReadSql(string SQL)
       {

       con = new SqlConnection(connectionstring);
       con.Open();(in this line the above error shows)
       cmd  = new SqlCommand(SQL,con);
       dr = cmd.ExecuteReader();
       return (dr);

   }