Error saving images to the external storage (sdcard)

Question

I'm trying to compress and store some photos in the external memory of an Android device using this method:

private String saveToExternalStorage(Bitmap bitmapImage, String nameImage, int compression) {
    String root = Environment.getExternalStorageDirectory().getAbsolutePath() + "/imagesDir";
    File file = new File(root, nameImage);
    FileOutputStream fos = null;
    try {
        fos = new FileOutputStream(file);
        // Use the compress method on the BitMap object to write image to the OutputStream
        bitmapImage.compress(Bitmap.CompressFormat.JPEG, compression, fos);
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        try {
            fos.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    return file.getAbsolutePath();
}

But I'm getting the following error:

E/AndroidRuntime: FATAL EXCEPTION: Thread-6
Process: com.universitat_autonoma_de_barcelona_geology_mineralogy.minescope, PID: 13633
java.lang.NullPointerException: Attempt to invoke virtual method 'void java.io.FileOutputStream.close()' on a null object reference
    at com.universitat_autonoma_de_barcelona_geology_mineralogy.minescope.Settings.saveToExternalStorage(Settings.java:219)
    at com.universitat_autonoma_de_barcelona_geology_mineralogy.minescope.Settings.access$100(Settings.java:39)
    at com.universitat_autonoma_de_barcelona_geology_mineralogy.minescope.Settings$3.run(Settings.java:120)
    at java.lang.Thread.run(Thread.java:923)

How can I save the photos in the sdcard?