绑定到嵌套在具有实际数据上下文设置的选项卡控件中的数据网格时存在问题

    namespace TestEval.ViewModel
    {
        class BuilderViewModel: ViewModelBase
        {
        private ObservableCollection<TabItem> tabs = new ObservableCollection<TabItem>();
        public ObservableCollection<TabItem> Tabs
        {
            get { return tabs; }
            set
            {
                tabs = value;
                OnPropertyChanged("Tabs");
            }
        }
        private int selectedIndex;
        public int SelectedIndex
        {
            get { return selectedIndex; }
            set
            {
                selectedIndex = value;
                SelectedEvaluation = Tabs[selectedIndex].DataContext as EvaluationViewModel;
                OnPropertyChanged("SelectedIndex");
            }
        }
        private EvaluationViewModel selectedEvaluation;
        public EvaluationViewModel SelectedEvaluation
        {
            get
            {
                return selectedEvaluation;
            }
            set
            {
                selectedEvaluation = value;
                OnPropertyChanged("SelectedEvaluation");
            }
        }
        private object selected;
        public object Selected
        {
            get { return selected; }
            set
            {
                if (value == null)
                {
                    return;
                }
                selected = value;
                if (selected is Group)
                {
                    selected = selected as Group;
                }
                else
                {
                    selected = selected as Question;
                }
            }
        }
        private RelayCommand addGroupCommand;
        public ICommand AddGroupCommand
        {
            get
            {
                if (addGroupCommand == null)
                {
                    addGroupCommand = new RelayCommand(p => this.AddGroup(), p => this.CanAddGroup);
                }
                return addGroupCommand;
            }
        }
        public BuilderViewModel()
        {
            Tabs.Add(new TabItem{ Header = "Company Evaluation", DataContext = new EvaluationViewModel("Company") });
            Tabs.Add(new TabItem { Header = "Customer Evaluation", DataContext = new EvaluationViewModel("Customer") });
        SelectedIndex = 0;
        }
        public bool CanAddGroup
        {
            get
            {
                 return SelectedEvaluation != null;
            }
        }
        void AddGroup()
        {
            SelectedEvaluation.Groups.Add(new Group(){Name = "GroupTest"});
        }
    }
}

namespace TestEval.ViewModel
{
    public class EvaluationViewModel : ViewModelBase
    {
        private ObservableCollection<Group> groups = new ObservableCollection<Group>();
        public ObservableCollection<Group> Groups
        {
            get { return groups; }
            set
            {
                groups = value;
                OnPropertyChanged("Groups");
            }
        }
        private string name;
        public string Name
        {
            get { return name; }
            set
            {
                name = value;
                OnPropertyChanged("Name");
            }
        }
        public EvaluationViewModel(string name)
        {
            this.Name = name;
        }
   }
}

namespace TestEval.Model
{
    public class Group : ObservableObject
    {
        private ObservableCollection<Question> questions = new ObservableCollection<Question>();
        public ObservableCollection<Question> Questions
        {
            get { return questions; }
            set
            {
                questions = value;
                OnPropertyChanged("Questions");
            }
        }
        private string name;
        public string Name
        {
            get { return name; }
            set
            {
                name = value;
                OnPropertyChanged("Name");
            }
        }  
    }
}

public class Question : ObservableObject
{
    private Guid id;
    private string content;
    private string answerType;
    private string answers;
    private bool isRequired;
    private string group;
    private bool showAnswers = false;
    public Guid Id
    {
        get { return id; }
        set { id = value; }
    }
    public string Content
    {
        get { return content; }
        set
        {
            content = value;
            OnPropertyChanged("Content");
        }
    }
    public string AnswerType
    {
        get { return answerType; }
        set
        {
            answerType = value;
            if (answerType == "SingleChoice" || answerType == "MultipleChoice")
            {
                ShowAnswers = true;
            }
            else
            {
                ShowAnswers = false;
            }
            OnPropertyChanged("AnswerType");
        }
    }
    public string Answers
    {
        get { return answers; }
        set
        {
            answers = value;
            OnPropertyChanged("Answers");
        }
    }
    public string Group
    {
        get { return group; }
        set
        {
            group = value;
            OnPropertyChanged("Group");
        }
    }
    public bool IsRequired
    {
        get { return isRequired; }
        set
        {
            isRequired = value;
            OnPropertyChanged("IsRequired");
        }
    }
    public bool ShowAnswers
    {
        get { return showAnswers; }
        set
        {
            showAnswers = value;
            OnPropertyChanged("ShowAnswers");
        }
    }
    public Question()
    {
    }
    public Question(string content, string answerType, string answers, bool isRequired)
    {
        this.Id = new Guid();
        this.Content = content;
        this.AnswerType = answerType;
        this.Answers = answers;
        this.IsRequired = isRequired;
    }
}