NullPointerException When ScreenShot

[bokala123]

When I use AShot to Shot Screen,error is exit.
My source:
private void captureImage(WebDriver driver, String filePath, float quality) throws IOException {
JPEGImageWriteParam param = new JPEGImageWriteParam(Locale.getDefault());
param.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
param.setCompressionQuality(quality);

// 150はミリ秒でスクロールを待つ時間、短すぎるとダメな模様
**ShootingStrategy ss = ShootingStrategies.viewportPasting(150); 

Screenshot screenshot = new AShot().shootingStrategy(ss).takeScreenshot(driver);**

ImageWriter writer = null;
try {
    writer = ImageIO.getImageWritersByFormatName("jpg").next();
    writer.setOutput(ImageIO.createImageOutputStream(new File(filePath)));
    writer.write(null, new IIOImage(screenshot.getImage(), null, null), param);
} finally {
    if (writer != null) {
        writer.dispose();
    }
}

}

when the project is running, ShootingStrategy ss.shottingArea are NULL!

how can i solve this issue