Getting null pointer exception in list Files method?

Question

I am trying to retrieve all DocumentFile[] using listFiles() method.But getting a null pointer exception how to eliminate this error and still get the files. This is the code which searches sd card root directory which is being passed as documentfile in method parameter.

  private List<DocumentFile> getListFiles2(DocumentFile docfile) {
        List<DocumentFile> inFiles = new ArrayList<>();
        Queue<DocumentFile> files = new LinkedList<>();


//  check error arising here
        files.addAll(Arrays.asList(docfile.listFiles()));


        while (!files.isEmpty()) {
            DocumentFile file = files.remove();
            if (file.isDirectory()) {
                files.addAll(Arrays.asList(file.listFiles()));

            } else if (file.getName().endsWith(".bmp")
|| file.getName().endsWith(".gif")
                    || file.getName().endsWith(".jpg") || file.getName().endsWith(".jpeg")
                    || file.getName().endsWith(".png") 
           ) {
                inFiles.add(file);

            }
        }
        return inFiles;
    }