Android : Android get all files in directory nullpointerexception

I'm trying to get all files of a specific directory on the SD card. Therefore I found this function

public static List<File> getListFiles(File parentDir) {
    ArrayList<File> inFiles = new ArrayList<File>();
    File[] files = parentDir.listFiles();

    for (File file : files) {
        if (file.isDirectory()) {
            inFiles.addAll(getListFiles(file));
        } else {
            if(file.getName().endsWith(".mp3")){
                inFiles.add(file);
            }
        }
    }

    return inFiles;
}

The problem is the following: If the folder is empty, it will throw a NullPointerException. How can I avoid this?

Permissions for read and write on external storage are set.