Derived attribute with AsTajson call in the custom service

I use .net5 and .netStandard from nuget. The structure of the solution is simple - class library with ecomodel and webapi project. I've registers custom service in the EcoSpace1.cs as it was advised by eco gurus long, long time ago.

public interface IMyService
{
    int Class1Count();
}

public class MyServiceClass : IMyService
{
    private IEcoServiceProvider ServiceProvider { get; set; }

    public MyServiceClass(IEcoServiceProvider serviceProvider)
    {
        ServiceProvider = serviceProvider;
    }
    public int Class1Count()
    {
        var v = ServiceProvider.GetEcoService<IOclService>().Evaluate("Class1.allInstances->size");
        return (int)v.AsObject;
    }
}

The rule for a webapi project's controller- inherit from the Controller class - due to the Swashbuckle.AspNetCore (swagger) usage. Swagger shows errors if I inherit a controller from the ModelDrivenControllerBase

But the code above is OK, the GET works perfect:

public class MySecondEcoServiceController : Controller
{
    [HttpGet]
    public int Get()
    {

        using (EcoSpace1 es=new EcoSpace1())
        {
            es.Active = true;
            int r = es.MyService.Class1Count();
            return r;
        }
    }

}

It works till I tried to add another method into the IMyService and to get json. I tried AsTajson by adding derived attribute to the Class1 with DerivationOcl.

Class1.allInstances->first.AsTaJson( 'SampleViewModel',  false )

or

self.AsTaJson( 'SampleViewModel',  false )

In the MyServiceClass the implementation of the Get is:

Class1 v = (Class1)ServiceProvider.GetEcoService<IOclService>().Evaluate("Class1.allInstances->first").AsObject;
json = v.Attribute2;

If I tried GET this value - exception:

Eco.FrameworkImpl.Ocl.EBoldOCLError: 'bughuntinfo internaleval:84  es seems fine.Object reference not set to an instance of an object.'

What is the proper way (or EcoService?) to get values returned by TaJson?

Not sure but please check that you have initiated use of ViewModels once for your EcoSpace type:

ViewModelDefinitionsInApplication.Init(_es);