public class TextDetectionTask extends AsyncTask<String, String, Void> {

    private static final String TAG = TextDetectionTask.class.getSimpleName();

    private Context context;
    private DatabaseReference dbRef;

    public TextDetectionTask(Context context, DatabaseReference dbRef) {
        this.context = context;
        this.dbRef = dbRef;
    }

    @Override
    protected Void doInBackground(String... strings) {
        String folderName = strings[0];

        File folder = new File(Environment.getExternalStorageDirectory().getPath() + "/images/" + folderName);
        File[] images = folder.listFiles();

        FirebaseVisionImage image = null;

        for(File file: images) { // images gets null

            final String imageName = file.getName().split(".jpg")[0];
        // ....

        }
        return null;
    }
}