connectionString Can not be null value

I have a problem with the connection string that sits inside the json configuration file , as usual :

1. i have created a class : applicationDbcontext
2. i made a DbSet {get;set;}
3. integrate it into the controller
4. add it to the services

I have read and try all the advices inserted in the related question : asp.net core 2.0 - Value cannot be null. Parameter name: connectionString But none of them works with me Here is my implementation , I hope someone can help me , thank you

A_ DB Context :

   public class ApplicationDbContext:DbContext
    {
        public ApplicationDbContext(DbContextOptions options) : base(options)
        {
            Database.EnsureCreated();
        }
        public DbSet<Person> Persons { get; set; }
    }
}

B_ App Settings :config.json and config.development.json

{
    "ConnectionString": {
        "testConnection": "Server=(localdb)\\mssqllocaldb;Database=mvc_db;Trusted_Connection=True;MultipleActiveResultSets=true"
    },
    "Logging": {
        "LogLevel": {
            "Default": "Warning"
        }
    },
    "AllowedHosts": "*"

}

C_ Services Injection

Startup :

  public Startup(IHostingEnvironment env)
        {
            var builder = new ConfigurationBuilder()
                .SetBasePath(env.ContentRootPath)
                .AddJsonFile("appsettings.Json")
                .AddJsonFile("appsettings.Development.Json", true)
                .AddEnvironmentVariables();

            Configuration = builder.Build();
        }

Configuration Services :

public void ConfigureServices(IServiceCollection services)
        {

            services.AddDbContext<ApplicationDbContext>(options => {
                options.UseSqlServer(Configuration.GetConnectionString("testConnection"));
            });
            services.Configure<CookiePolicyOptions>(options =>
            {
                // This lambda determines whether user consent for non-essential cookies is needed for a given request.
                options.CheckConsentNeeded = context => true;
                options.MinimumSameSitePolicy = SameSiteMode.None;
            });


            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
        }

D_ Injection Into Controllers :

 private readonly ApplicationDbContext _db;
        public PersonController(ApplicationDbContext db)
        {
            _db = db;
        }

        IEnumerable<Person> People { get; set; }
        public IActionResult Index()
        {
            People = _db.Persons.ToList();
            return View(People);
        }

I have tried to insert the whole connectionString inside the configuration.GetConnectionString("Here"); And To change the location of the connection string from up to down and vice verse. But nothing fix the return null value problem of the connectionString . Any Help please Thanks