Getting a nullpointer exception on getApiClient

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);

I'm getting a error on apiClient as : Cant invoke getApiClient() on a null object reference

 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;
}

This is the code from where i invoke the method getApiClient