How to properly set A Retrofit response into a MutableLiveData so that it becomes observable

Question

• What I'm trying to do : I am trying to show a list of cities in a RecyclerView in the MainActivity and whenever a city in the list is clicked a WeatherDetailsActivity should fetch a response from the OpenWeather's API using Retrofit. I'm using MVVM architecture.
• What I've managed to do so far : I've got the list showing up perfectly and even the response is fetched properly according to a POJO model, but when I click on a city it crashes due to NullPointerException.
• TL;DR : I don't understand why the return weatherResult; in the WeatherRepository.java always return null.

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• Code:

WeatherRepository.java

public class WeatherRepository {
    final String TAG = this.getClass()
                           .getSimpleName();

    private static final String WEATHER_API_URL = "https://api.openweathermap.org/";
    private static WeatherRepository weatherRepository;
    private APIService apiService;
    MutableLiveData weatherResult;


    private WeatherRepository() {
        Retrofit retrofit = new Retrofit.Builder().baseUrl(WEATHER_API_URL)
                                                  .addConverterFactory(GsonConverterFactory.create())
                                                  .build();
        apiService = retrofit.create(APIService.class);
    }

    public synchronized static WeatherRepository getInstance() {
        if (weatherRepository == null) {
            return new WeatherRepository();
        }
        return weatherRepository;
    }

    public MutableLiveData<WeatherResult> getWeatherData(String city, String appid) {
        apiService.getWeatherData(city, appid)
                  .enqueue(
                          new Callback<WeatherResult>() {
                              @Override
                              public void onResponse(Call<WeatherResult> call, Response<WeatherResult> response) {

                                  if (response.isSuccessful()) {
                                      Log.i(TAG, "onResponse: " + response.body()
                                                                          .toString());
                                      //response.body() gets the value perfectly.
                                      weatherResult.setValue(response.body());
                                  }
                              }
                              @Override
                              public void onFailure(Call<WeatherResult> call, Throwable t) {
                                  t.printStackTrace();
                                  Log.d(TAG, "onFailure: " + t.getMessage());
                              }
                          });

        return weatherResult; //this always return null
    }
}

---

WeatherViewModel.java

public class WeatherViewModel extends ViewModel
{
    private String TAG = this.getClass().getSimpleName();
     private MutableLiveData<WeatherResult> data = new MutableLiveData<>();


    private String APP_ID = /*secret api key*/;


    public void getWeather(String city)
    {
        data.setValue(WeatherRepository.getInstance().getWeatherData(city,APP_ID).getValue()); // this line throws NullPointerException.
        Log.d(TAG, "WeatherViewModel: "+(data.toString()));
    }

    public LiveData<WeatherResult> getCityDetails()
    {
        return data;
    }

}

---

APIService.java

public interface APIService {

    //Weather API
    //http://samples.openweathermap.org/data/2.5/weather?q=London&appid={APP_ID}

    @GET("data/2.5/weather?")
    Call<WeatherResult> getWeatherData(@Query("q") String city,@Query("appid") String appID);

}

---

WeatherDetails.java

public class WeatherDetails extends AppCompatActivity {

    WeatherViewModel viewModel;
    TextView textView;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.weather_details);

        textView = findViewById(R.id.weather_info_text);
        viewModel = ViewModelProviders.of(this).get(WeatherViewModel.class);
    }

    @Override
    protected void onStart() {
        super.onStart();
        viewModel.getCityDetails().observe(this, new Observer<WeatherResult>() {
            @Override
            public void onChanged(WeatherResult weatherResult) {
                textView.setText(weatherResult.toString()); //this line throws NullPointerException
            }
        });
    }
}

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All I want is to return the Retrofit response and set it into the MutableLiveData<WeatherResult> so that it is observed inside the WeatherDetails.java and the UI is updated accordingly.

I don't get why the return weatherResult; in the WeatherRepository.java always returns null

Now I'm not sure how to proceed, I'm at the end of the ropes so I'm asking it here.

Incase you want to look at the entire code here is the code on github. Thank you for every and any help.

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UPDATE I did the following changes to resolve NullPointerException and the crash is fixed. link to diff

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UPDATE 2 Here is the final working code, what did it for me was to get rid of a MutableLiveData in my ViewModel class and instead use a public method in my ViewModel as a wrapper to call the repository service call to the API server. Here is the link to the diff. Thank you to everyone who helped.