如何解决尝试在空对象引用上调用虚方法'void android.arch.lifecycle.LiveData.observe？(语言-java)

我正在使用Android架构组件，并尝试从LiveData实例化viewmodel和Observe数据，然后出现这个问题

Attempt to invoke virtual method 'void androidx.lifecycle.LiveData.observe(androidx.lifecycle.LifecycleOwner, androidx.lifecycle.Observer)' on a null object reference

这是我的调用Observe的地方,也就是报错的地方

public class CrimeListFragment extends Fragment {

    private RecyclerView mRecyclerView;
    private CrimeAdapter mAdapter;
    private boolean  mSubtitleVisible;
    private LiveData<List<Crime>> mListLiveData;
    private CrimeViewModel mCrimeViewModel;
    //这里就只贴出了问题的区域

    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view=inflater.inflate(R.layout.fragment_crime_list,container,false);
        mRecyclerView=view.findViewById(R.id.crime_recycle_ListView);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
        mCrimeViewModel=new ViewModelProvider(this).get(CrimeViewModel.class);
        mListLiveData=mCrimeViewModel.getListLiveData();
        mListLiveData.observe(this.getViewLifecycleOwner(), new Observer<List<Crime>>() {
        //这里报错
            @Override
            public void onChanged(List<Crime> crimes) {
                mAdapter.setCrimes(crimes);
                updateUi();
            }
        });
        updateUi();
        return view;
    }
}

这里是我Repository类和Runnable

//这个是Repository类
public class CrimeRepository {

    private final CrimeDAO mCrimeDAO;

    public CrimeRepository(Context context){
        mCrimeDAO= CrimeDatabase.getCrimeDataBase(context).getCrimeDao();
    }

    public LiveData<List<Crime>> getCrimeListLive() {
        MyRunnable myRunnable=new MyRunnable("SelectAllCrimeLive");
        Executors.newCachedThreadPool().execute(myRunnable);
        return myRunnable.getListLiveData();
    }

   public List<Crime> selectListCrime(){
        MyRunnable myRunnable=new MyRunnable("SelectAllCrime");
        Executors.newCachedThreadPool().execute(myRunnable);
        return myRunnable.getCrimes();
    }

//这是我自定义实现接口Runnable的类，CrimeRepository 的私有类

 private class MyRunnable implements Runnable{

      private LiveData<List<Crime>> mListLiveData;
        @Override
        public void run() {
            mListLiveData=mCrimeDAO.selectAllCrimeLive();
  }
}

这是我的ViewModel

public class CrimeViewModel extends ViewModel {

    private final CrimeRepository       mCrimeRepository;
    public CrimeViewModel(@NonNull Application application){
        mCrimeRepository=new CrimeRepository(application);
    }
  public LiveData<List<Crime>> getListLiveData(){
        return mCrimeRepository.getCrimeListLive();
    }
}

这是截图：

在这里加一个日志，看有没有被调用到
public CrimeViewModel(@NonNull Application application){
mCrimeRepository=new CrimeRepository(application);
}

public LiveData<List<Crime>> getCrimeListLive() {
        MyRunnable myRunnable=new MyRunnable("SelectAllCrimeLive");
        Executors.newCachedThreadPool().execute(myRunnable);
        return myRunnable.getListLiveData();
    }

这里不对吧，线程池会在子线程执行任务，而getCrimeListLive所在的线程就会继续进行，myRunnable.getListLiveData()会直接返回空，因为另一边线程还没有执行结束。
这是多线程问题，一个线程没有等另一个线程的结果就返回了，时序是这样的：
【线程1】新建myRunnable实例，此时mListLiveData为null;
【线程1】开启【线程2】执行myRunnable的run()方法
【线程1】返回myRunnable.getListLiveData();此时mListLiveData为null;
【线程2】run()执行结束，为mListLiveData赋值。