Csharp ProjectToValue 不能为空. (参数来源)

以下代码导致Value cannot be null. (Parameter 'source')异常.它是由.ProjectTo引起的，更具体地说，它是由DisciplineDto中的DisciplineHours引起的.很难说是什么原因造成的，但我该如何追溯呢？

它可能是像smt一样的.ThenInclude(discipline => discipline.DisciplineHours).

失败:学业进度表.应用学科.查询.GetDisciplinesByFlowIdQuery[0]

using Response = List<DisciplineDto>;

public class GetDisciplinesByFlowIdQuery : IRequest<Response>
{
    public int FlowId { get; set; }
}

public class GetDisciplinesByFlowIdQueryHandler : IRequestHandler<GetDisciplinesByFlowIdQuery, Response>
{
    private readonly IApplicationDbContext _context;
    private readonly IMapper _mapper;

    public GetDisciplinesByFlowIdQueryHandler(IApplicationDbContext context, IMapper mapper)
    {
        _context = context;
        _mapper = mapper;
    }

    public Task<Response> Handle(GetDisciplinesByFlowIdQuery request, CancellationToken cancellationToken)
    {
        var flow = _context.Flows
            .Include(f => f.Disciplines)
            .SingleOrDefault(x => x.Id == request.FlowId);

        if (flow == null)
        {
            throw new NotFoundException(nameof(Flow), request.FlowId);
        }

        var result = flow.Disciplines
            .AsQueryable()
            .ProjectTo<DisciplineDto>(_mapper.ConfigurationProvider)
            .ToList();

        return Task.FromResult(result);
    }
}

public class DisciplineDto : IMapFrom<Discipline>
{
    public int Id { get; set; }

    public string ShortName { get; set; }

    public string FullName { get; set; }

    public string DisciplineCode { get; set; }

    public int SemesterNumber { get; set; }

    public bool IsRequired { get; set; }

    public string CurriculumCode { get; set; }

    public int CurriculumVersion { get; set; }

    public DepartmentDto Department { get; set; }
    public PeriodDto Period { get; set; }
    public List<DisciplineHourDto> DisciplineHours { get; set; } // Caused by this line
}

public class Discipline
{
    public int Id { get; set; }
    public string ShortName { get; set; }
    public string FullName { get; set; }
    public string DisciplineCode { get; set; }
    public int SemesterNumber { get; set; }
    public bool IsRequired { get; set; }
    public string CurriculumCode { get; set; }
    public int CurriculumVersion { get; set; }

    public int DepartmentId { get; set; }
    public Department Department { get; set; }
    
    public int PeriodId { get; set; }
    public Period Period { get; set; }

    public List<Flow> Flows { get; set; }
    public List<DisciplineHour> DisciplineHours { get; set; }
    public List<ClassActivity> ClassActivities { get; set; }
}

public class Flow
{
    public int Id { get; set; }
    public int PeriodId { get; set; }
    public Period Period { get; set; }

    public List<Discipline> Disciplines { get; set; }
    public List<Subgroup> Subgroups { get; set; }
    public List<ClassActivity> ClassActivities { get; set; }
}

public class DisciplineHour
{
    public int DisciplineId { get; set; }
    public Discipline Discipline { get; set; }

    public int ClassActivityTypeId { get; set; }
    public ClassActivityType ClassActivityType { get; set; }

    public decimal Hours { get; set; }
}

public class DisciplineHourDto : IMapFrom<DisciplineHour>
{
    public int DisciplineId { get; set; }

    public int ClassActivityTypeId { get; set; }

    public decimal Hours { get; set; }

    public string DisciplineName { get; set; }

    public string ClassActivityTypeName { get; set; }

    public string FullName { get; set; }

    public void Mapping(Profile profile)
    {
        profile.CreateMap<DisciplineHour, DisciplineHourDto>()
            .ForMember(dto => dto.DisciplineName, opt => opt.MapFrom(sh => sh.Discipline.FullName))
            .ForMember(dto => dto.ClassActivityTypeName, opt => opt.MapFrom(sh => sh.ClassActivityType.ShortName))
            .ForMember(dto => dto.FullName, opt => opt.MapFrom(sh => $"{sh.ClassActivityType.FullName} {sh.Discipline.ShortName}"));
    }
}