Adding custom logics to Mapster's mapping, possibly with AfterMapping?

Question

The Question

After mapping Input to Output, the returned Input must have an incrementing number in its Entries[i].Id, (i.e. Entries[0].Id = 1, Entries[1].Id = 2, Entries[2].Id = 3, ...). So I tried to put it in AfterMapping:

[Mapper]
public interface IMyMapper {
    Output Map(Input userTableTemplate);
}

public class RegisterMyMapper : IRegister {
    public void Register(TypeAdapterConfig config) {
        config.NewConfig<Input, Output>()
            .Map(output => output.OutputName, input => input.Name)
            .AfterMapping(output => {
                foreach (var pair in output.Entries.Select((value, index) => new {value = value, index = index})) {
                    pair.value.Id = pair.index + 1;
                }
            });
    }
}

public class Output {
    public string OutputName { get; set; }
    public IEnumerable<Entry> Entries { get; set; }
}

public class Entry { public int Id { get; set; } }

public class Input { public string Name { get; set; } }

But when running _myMapper.Map(myInput) I'm getting a null reference exception, because the private Action<Output> Action1 member of the generated code (public partial class MyMapper : IIMyMapper) is null:

public partial class MyMapper : IMyMapper
{
    private Action<Output> Action1;
    
    public Output Map(Input p1)
    {
        if (p1 == null)
        {
            return null;
        }
        Output result = new Output();
        
        result.OutputName = p1.Name;
        Action1.Invoke(result);
        return result;
    }
}

Current solution

Switched from AfterMapping to AfterMappingInline:

config.NewConfig<Input, Output>()
    .Map(output => output.OutputName, input => input.Name)
    .AfterMappingInline(output => output.Entries.SetIdValues())

And:

public static class MethodExtensions {
    public static void SetIdValues(this IEnumerable<Entry> entries) {
        foreach (var pair in entries.Select((value, index) => new {value = value, index = index}))
            pair.value.Id = pair.index + 1;
    }
}

...was hoping to find more elegant solution...