Error in calling the migration dimension tables and updating the database

I wrote a C# project with the change that I used migration

To be able to use sturtup in the program:

public static IHostBuilder CreateHostBuilder(string[] args)
    => Host.CreateDefaultBuilder(args)
           .ConfigureWebHostDefaults(
              webBuilder => webBuilder.UseStartup<startup>()
           );

, and commands in Startup:

public class Startup
{
    public IConfiguration Configuration { get; set; }

    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }
}


public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<readscalecontext>(options =>
    {
        options.UseSqlServer(Configuration.GetConnectionString("ReadScaleConnection"));
    });
}

The tables were all created correctly with the migration command. I just want to know why the tables are not called and it returns null. The example of my first command is:

List<accounttype> GetAllAccountType();
and used this:
 dgvOperators.DataSource = _operatorService.GetAllOperatore();

Thank you very much for your guidance

Error:

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

db was null.

What I have tried:

i use enabale-migration and update-database