СonnectionString is null

{

  "ConnectionStrings": {

    "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=myDB;Trusted_Connection=True;MultipleActiveResultSets=true"

  }

 }

public Startup(IHostingEnvironment env)

        {

            var builder = new ConfigurationBuilder()

                .SetBasePath(env.ContentRootPath)

                .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)

                .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true);

            builder.AddEnvironmentVariables();

            Configuration = builder.Build();

        }

Microsoft.Extensions.Configuration

Microsoft.Extensions.Configuration.Json