[Solved] ASP.Net ConnectionString Error: Object reference not set to an instance of an object

There is an error while running my code:

local host says undefined: 

an exception is occure :

An exception of type 'System.NullReferenceException' occurred in WebApplication13.dll but was not handled in user code

Additional information: Object reference not set to an instance of an object.

web config file:

<?xml version="1.0" encoding="utf-8"?>
<!--
  For more information on how to configure your ASP.NET application, please visit
  http://go.microsoft.com/fwlink/?LinkId=169433
  -->
<configuration>
  <connectionStrings>
    <add name="dbcs" connectionString="data source=DESKTOP-9OM984O;initial catalog=Airbluedb;integrated security=true"/>
  </connectionStrings>
  <system.web>
    <compilation debug="true" targetFramework="4.5" />
    <httpRuntime targetFramework="4.5" />
  </system.web>
</configuration>

C# CODE

namespace WebApplication13
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        [WebMethod]
        public static List<object> GetChartData()
        {
            string query = "SELECT [time],[average] FROM [dbo].[graph] WHERE ipaddress='192.168.15.167'";
            string constr = ConfigurationManager.ConnectionStrings["dbsc"].ConnectionString;
            List<object> chartData = new List<object>();
            chartData.Add(new object[]
            {
                "time", "average"
            });
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand(query))
                {
                    cmd.CommandType = CommandType.Text;
                    cmd.Connection = con;
                    con.Open();
                    using (SqlDataReader sdr = cmd.ExecuteReader())
                    {
                        while (sdr.Read())
                        {
                            chartData.Add(new object[]
                            {
                                sdr["time"], sdr["average"]
                            });
                        }
                    }
                    con.Close();
                    return chartData;
                }
            }
        }
    }
}

please help me as i am stucking with this error.

i dont know whats wrong with this code or why its not giving input