#vulnerable code 
                                private void buildIndex(final List<String> index, final File root, final String pathBase) {
        for (File file : root.listFiles()) {
            if (file.isDirectory()) {
                index.add(pathBase + file.getName());
                buildIndex(index, file, pathBase + file.getName() + "/");
            }
        }
    } 
                            #location 2 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private void buildIndex(final List<String> index, final File root, final String pathBase) {
        File[] files = root.listFiles();
        if (files != null) for (File file : files) {
            if (file.isDirectory()) {
                index.add(pathBase + file.getName());
                buildIndex(index, file, pathBase + file.getName() + "/");
            }
        }
    }