java.lang.NullPointerException的原因是什么？使用Apache-Flink

如何解决java.lang.NullPointerException的原因是什么？使用Apache-Flink

我需要使用tessearct库（tess4j）从bufferImage中提取文本，但是函数会抛出NullPointerException。

PS：当我在flatMap范围之外执行函数getimageText()时，该函数运行正常

public String getImgText(BufferedImage bufferImage) throws TesseractException {
        ITesseract instance = new Tesseract();
        try
        {
            if (instance == null){
                System.out.println("instance is null");
            }
            String imgText = instance.doOCR(bufferImage);
            return imgText;
        }
        catch (TesseractException e)
        {
            return "Error while reading image: "+e.getMessage();
        }
    }

e_stream.flatMap(new FlatMapFunction<BufferedImage,String>() {
                @Override
                public void flatMap(BufferedImage iBuffer,Collector<String> out) throws Exception{
                    TextExtraction ocr = new TextExtraction();
                    String cv = ocr.getImgText(iBuffer);
                    out.collect(cv);
                }
            }).print();