private Observable<String> stringObservable(){
       return Observable
                .just("1","2","3")
                .flatMap((Function<String, ObservableSource<String>>) s -> {
                    if (s.equals("3")){
                        throw new NullPointerException();
                    }
                    return Observable.just("1");
                })
                .retryUntil(() -> true)
                .doOnError(it -> Log.e(TAG, "showSnackbar: " ))
                .onErrorResumeNext(stringObservable());
    }