Null pointer appears for rxjava nested network access

the text in the diagram needs to be identified through ocr in the project, and the third-party ocr library is referenced, but there is a problem in the nesting of rxjava, and the key part of the code is pasted

Disposable disposable = Observable.just(OCR())
                        .observeOn(AndroidSchedulers.MainThread())
                        .subscribeOn(schedulers.newThread())
                        .flatMap(new Function<Rsult,ItemsBean>(){
                            ............
                        })
                        .subscribe(new Consumer(){
                            ............
                        })

where the ocr method itself is accessed through the network and returns the Request instance, probably posting the code of the ocr method

public static Result OCR(){
    ImageClient imageClient = new ImageClient();
    String result = imageClient.OCR(**,**,**);//
    Result resultEntity = gson.fromJson(result);
    return resultEntity;
}

after running, it is found that the result instance obtained in flatMap is empty, and then part of the code is changed to something like this

Disposable disposable = Observable.just(imageClient.OCR(**,**,**))
                        .observeOn(AndroidSchedulers.MainThread())
                        .subscribeOn(schedulers.newThread())
                        .map(new Function(){
                            ............
                        })
                        .flatMap(new Function<Rsult,ItemsBean>(){
                            ............
                        })
                        .subscribe(new Consumer(){
                            ............
                        })

then report network access in the main thread, what is the reason for this, and how to do this similar combination of network access nesting and rxjava?

Disposable disposable = Observable.fromCallable( @Override public Object call(){ return ..... }) .observeOn(AndroidSchedulers.MainThread()) .subscribeOn(schedulers.newThread()) .flatMap(new Function<Rsult,ItemsBean>(){ ............ }) .subscribe(new Consumer(){ ............ })