ASP.Net MVC Error: System.Configuration.ConnectionStringSettingsCollection.this[string].get returned null

Im getting error at HomeController as

System.NullReferenceException: 'Object reference not set to an instance of an object.'

System.Configuration.ConnectionStringSettingsCollection.this[string].get returned null.

can you please help me in solving this?

HomeController.cs 

using jQuery_AJAX_WebAPI_MVC.Models;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using jQuery_AJAX_MVC.Models;
using System.Runtime.Remoting.Contexts;
using Microsoft.JScript;
using System.Data.SqlClient;
using System.Configuration;

namespace jQuery_AJAX_WebAPI_MVC.Controllers
{
    public class HomeController : Controller
    {
        // GET: Home
        public ActionResult Index()
        {
            return View();
        }

        public ActionResult HomePage()

        {
            return View();
        }
        private List<SelectListItem> GetEmployeeListSelectListItem(List<Department> dep)
        {
            List<SelectListItem> items = new List<SelectListItem>();
            foreach (Department d in dep)
            {
                items.Add(new SelectListItem
                {
                    Text = d.DEPNAME,
                    Value = d.DEPID.ToString()
                });
            }
            return items;
        }
        public List<Department> GetEmployeeList()
        {
            List<Department> items = new List<Department>();
            string constr = ConfigurationManager.ConnectionStrings["Constring"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                string query = " SELECT DEPID,DEPNAME,MANAGER FROM Department";
                using (SqlCommand cmd = new SqlCommand(query))
                {
                    cmd.Connection = con;
                    con.Open();
                    using (SqlDataReader sdr = cmd.ExecuteReader())
                    {
                        while (sdr.Read())
                        {
                            items.Add(new Department
                            {
                                Text = sdr["DEPNAME"].ToString(),
                                Value = sdr["DEPID"].ToString()
                            });
                        }
                    }
                    con.Close();
                }
            }

            return items;
        }
    }
}

web.config

<connectionStrings>
    <add name="EmployeeEntities10" connectionString="metadata=res://*/Models.ModelData.csdl|res://*/Models.ModelData.ssdl|res://*/Models.ModelData.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=LENOVO-PC\MSSQL_SERVER;initial catalog=Employee;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework&quot;" providerName="System.Data.EntityClient" />
</connectionStrings>

in connection string I can see

Data Source=LENOVO-PC\MSSQL_SERVER;Initial Catalog=Employee;Integrated Security=True;MultipleActiveResultSets=True;Application Name=EntityFramework