android室中的null异常

Question

我正在使用Dao SQLite验证来自用户的电子邮件。发送给Dao的电子邮件是成功的，但是当我尝试从Dao的返回中获取数据时，返回结果为NULL。

这是Dao界面：

@Dao
public interface NoteDao {

@Query("SELECT * FROM user_table WHERE email = :email")
    Note get_UserInfo(String email);
}

这是存储库：

public class NoteRepository {
    private NoteDao noteDao;
    private LiveData<List<Note>> allNotes;

    public NoteRepository(Application application) {
        NoteDatabase database = NoteDatabase.getDatabase(application);
        noteDao = database.noteDao();
        allNotes = noteDao.getAllNotes();
    }
public Note get_UserInfo(String email){
        return noteDao.get_UserInfo(email);
    }
}

这是ViewModel类：

public class NoteViewModel extends AndroidViewModel {
    private NoteRepository repository;
    private LiveData<List<Note>> allNotes;

    public NoteViewModel(@NonNull Application application) {
        super(application);
        repository = new NoteRepository(application);
        allNotes = repository.getAllNotes();
    }

public Note get_UserInfo(String email) {
        return repository.get_UserInfo(email);
    }

}

这是寄存器类：

public class Register extends Fragment{
    private NoteViewModel noteViewModel;
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState){
        noteViewModel = ViewModelProviders.of(this).get(NoteViewModel.class);
Note userInfo = noteViewModel.get_UserInfo(email);
                String user_email = userInfo.getEmail();

                if (user_email != null){
                    Note note = new Note();
                    note.setEmail(email);
                    note.setUsername(username);
                    note.setPassword(new_password);

                    noteViewModel.insert(note);
                    Toasty.success(getActivity(), "Register Success", Toasty.LENGTH_SHORT).show();
                    UserAccount.fragmentManager.beginTransaction().replace(R.id.user_fragment, new Login()).commit();
                }else {
                    Toasty.error(getActivity(), "Please enter a valid Email", Toasty.LENGTH_SHORT).show();
                }
}
}