System Null Reference Exception

Hello, i have solved a challenge of DB connection. Iam now getting this error : system null reference exception when i run this code :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;
namespace eMarksCollectorLib
{
public class dbClass
{
private SqlConnection con;
private SqlCommand com;
private SqlDataAdapter adapter;
private DataSet _data;
public SqlDataReader reader;
public DataSet Data
{
get { return _data; }
set { _data = value; }
}
public dbClass()
{
//con = new SqlConnection(ConfigurationManager.ConnectionStrings["con"].ConnectionString); <<<< this is the line of code that has the problem>>>>
adapter = new SqlDataAdapter("", con);
com = new SqlCommand("", con);
_data = new DataSet();
}
public void Execute(string sql)
{
if (con.State == ConnectionState.Closed) con.Open();
com = new SqlCommand(sql, con);
com.ExecuteNonQuery();
con.Close();
}
public int ExecuteReturnID(string sql)
{
if (con.State == ConnectionState.Closed) con.Open();
com = new SqlCommand(sql, con);
return Convert.ToInt32(com.ExecuteScalar());
}
public DataTable Select(string sql, string table)
{
adapter.SelectCommand.CommandText = sql;
adapter.SelectCommand.Connection = con;// = sql;
if (_data.Tables.Contains(table))
{
_data.Tables.Clear();
}
adapter.Fill(_data, table);
con.Close();
return _data.Tables[table];
}
public string ExecuteScalar(string sql)
{
string results = "";
if (con.State == ConnectionState.Closed) con.Open();
com = new SqlCommand(sql, con);
results = com.ExecuteScalar().ToString();
con.Close();
return results;
}
public string ExecuteMyReader(string sql)
{
string results = "";
if (con.State == ConnectionState.Closed) con.Open();
com = new SqlCommand(sql, con);
reader = com.ExecuteReader(CommandBehavior.CloseConnection);
while (reader.Read())
{
if (reader[0] != DBNull.Value) results = reader[0].ToString();
}
reader.Close();
con.Close();
return results;
}
public DataTable GetData(string sql, SqlParameter[] sqlParameter)
{
DataSet ds = new DataSet();
SqlCommand cmd = new SqlCommand(sql, con);
if (sqlParameter != null)
{
cmd.Parameters.AddRange(sqlParameter);
}
SqlDataAdapter da = new SqlDataAdapter(cmd);
da.Fill(ds);
return ds.Tables[0];
}
}
}

the challenge with the code appears at this section :

public dbClass()
{
con = new SqlConnection(ConfigurationManager.ConnectionStrings["con"].ConnectionString);
adapter = new SqlDataAdapter("", con);
com = new SqlCommand("", con);
_data = new DataSet();
}