在getApiClient上获取nullpointer异常

Question

 public void doSignUp(OnAddUpdateListener<SignUpValidationError> listListener, SignUpRequest request) {
    mListener = listListener;
    LeafApiClient apiClient = LeafApplication.getInstance().getApiClient();
    LeafService service = apiClient.getService(LeafService.class);
    final Call<SignUpResponse> model = service.signup(request);
    ResponseWrapper<SignUpResponse> wrapper = new ResponseWrapper<>(model);

我在apiClient上遇到错误：无法在空对象引用上调用getApiClient（）

 private LeafApiClient apiClient;
private static LeafApplication sInstance;

@Override
public void onCreate() {
    super.onCreate();
    AppCompatDelegate.setCompatVectorFromResourcesEnabled(true);
    sInstance = this;


 public static LeafApplication getInstance(){
    return sInstance;
}

public synchronized LeafApiClient getApiClient(){
    if (apiClient == null){
        apiClient = LeafApiClient.getInstance(getApplicationContext());
    }
    return apiClient;
}

这是我调用方法getApiClient

的代码