将其传递给ViewModel时，Livedata对象无效

Question

我正在Android Studio制作一个项目，在那里我从API中获取了一个对象列表。 不幸的是，我对Livedata有一个问题，因为它将其从存储库传递到ViewModel。

在存储库类中，它称为API，解析响应并设置Livedata的值：

    private final Dao dao;
    private static Repository instance;
    private final MutableLiveData<Fare> searchedFare;
    @Nullable
    private final MutableLiveData<List<Arrival>> searchedArrivals;

    private Repository(){
        dao = Dao.getInstance();
        searchedFare = new MutableLiveData<>();
        searchedArrivals = new MutableLiveData<>() ;

    }

    public static Repository getInstance(){
        if(instance==null){
            instance = new Repository();
        }
        return instance;
    }

    public MutableLiveData<List<Arrival>> getReceivedArrivals() {return searchedArrivals;}

    public void searchForArrivals(){
        TaxiApiMock taxiApiMock = ServiceGenerator.getTaxiApi();
        Call<List<Arrival>> call = taxiApiMock.getArrivals();
        call.enqueue(new Callback<List<Arrival>>() {
            @EverythingIsNonNull
            @Override
            public void onResponse(Call<List<Arrival>> call, Response<List<Arrival>> response) {
                if(response.isSuccessful())
                    searchedArrivals.setValue(response.body());
                Log.d("Retrofit","Messages successfully received!");
                Log.d("Retrofit", new Gson().toJson(response.body()));
                Log.d("Retrofit", "Message successfully parsed!");

                for (Arrival arrival: Objects.requireNonNull(searchedArrivals.getValue())) {
                    Log.d("Retrofit",arrival.toString());
                }
                Log.d("Retrofit", searchedArrivals.toString());
                Log.d("Retrofit", searchedArrivals.getValue().toString());
                Log.d("Retrofit", searchedArrivals.getValue().get(0).toString());

            }

            @EverythingIsNonNull
            @Override
            public void onFailure(Call<List<Arrival>> call, Throwable t) {
                Log.i("Retrofit","Something went wrong! :(");
            }
        });
    }

一切按计划进行，直到我将其传递给ViewModel，在那里我获得了NullPointer。

public void searchForArrival(){
        repository.searchForArrivals();
        Log.d("VM", repository.getReceivedArrivals().toString());
        Log.d("VM", repository.getReceivedArrivals().getValue().toString());
        if(mArrival.getValue()==null)
            Log.d("VM", "list is null");
        else{
        Log.d("Retrofit", mArrival.toString());
        Log.d("Retrofit", mArrival.getValue().toString());
        Log.d("Retrofit", mArrival.getValue().get(0).toString());}
    }

这是LogCat在ViewModel类中打印时的外观：

    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
        at com.example.taxiapp.ui.timetable.TimetableViewModel.searchForArrival(TimetableViewModel.java:47)
        at com.example.taxiapp.ui.timetable.TimetableFragment$1.onClick(TimetableFragment.java:37)...

在此处（完美）存储库类中的外观：

2022-05-16 01:47:56.901 10487-10487/com.example.taxiapp D/Retrofit: Messages successfully received!
2022-05-16 01:47:56.905 10487-10487/com.example.taxiapp D/Retrofit: [{"cancelled":false,"date":"14.05.22","messages":"Lorem Ipsum","name":"Bus 100","origin":"CPH-Lufthavn","rtTrack":0,"stop":"Horsens","time":"21:37","track":0,"type":"IC"},{"cancelled":false,"date":"14.05.22","messages":"Lorem Ipsum","name":"Bus 100","origin":"Esbjerg","rtTrack":0,"stop":"Horsens","time":"23:37","track":0,"type":"REG"},{"cancelled":false,"date":"14.05.22","messages":"Lorem Ipsum","name":"Bus 100","origin":"Randers","rtTrack":0,"stop":"Horsens","time":"22:37","track":0,"type":"IC"}]
2022-05-16 01:47:56.906 10487-10487/com.example.taxiapp D/Retrofit: Message successfully parsed!
2022-05-16 01:47:56.906 10487-10487/com.example.taxiapp D/Retrofit: Arrival{name='Bus 100', type='IC', stop='Horsens', time='21:37', date='14.05.22', track=0, realTime='null', realDate='null', realTrack=0, origin='CPH-Lufthavn', cancelled=false, messages='Lorem Ipsum'}
2022-05-16 01:47:56.906 10487-10487/com.example.taxiapp D/Retrofit: Arrival{name='Bus 100', type='REG', stop='Horsens', time='23:37', date='14.05.22', track=0, realTime='null', realDate='null', realTrack=0, origin='Esbjerg', cancelled=false, messages='Lorem Ipsum'}
2022-05-16 01:47:56.906 10487-10487/com.example.taxiapp D/Retrofit: Arrival{name='Bus 100', type='IC', stop='Horsens', time='22:37', date='14.05.22', track=0, realTime='null', realDate='null', realTrack=0, origin='Randers', cancelled=false, messages='Lorem Ipsum'}
2022-05-16 01:47:56.906 10487-10487/com.example.taxiapp D/Retrofit: androidx.lifecycle.MutableLiveData@49b4d1d
2022-05-16 01:47:56.907 10487-10487/com.example.taxiapp D/Retrofit: [Arrival{name='Bus 100', type='IC', stop='Horsens', time='21:37', date='14.05.22', track=0, realTime='null', realDate='null', realTrack=0, origin='CPH-Lufthavn', cancelled=false, messages='Lorem Ipsum'}, Arrival{name='Bus 100', type='REG', stop='Horsens', time='23:37', date='14.05.22', track=0, realTime='null', realDate='null', realTrack=0, origin='Esbjerg', cancelled=false, messages='Lorem Ipsum'}, Arrival{name='Bus 100', type='IC', stop='Horsens', time='22:37', date='14.05.22', track=0, realTime='null', realDate='null', realTrack=0, origin='Randers', cancelled=false, messages='Lorem Ipsum'}]
2022-05-16 01:47:56.907 10487-10487/com.example.taxiapp D/Retrofit: Arrival{name='Bus 100', type='IC', stop='Horsens', time='21:37', date='14.05.22', track=0, realTime='null', realDate='null', realTrack=0, origin='CPH-Lufthavn', cancelled=false, messages='Lorem Ipsum'}