Serializacja XML "java.lang.NullPointerException"

 <book id="100E27">
        <title>100 sposobów na Excel 2007 PL. Tworzenie funkcjonalnych arkuszy</title>
        <authors>
            <author>David Hawley</author>
            <author>Raina Hawley</author>
        </authors>
        <isbn>978-83-246-1331-1</isbn>
        <publisher>Helion</publisher>
        <year>2008</year>
        <pages>368</pages>
    </book>

try {
			JAXBContext context = JAXBContext.newInstance(BookList.class);
			Unmarshaller um = context.createUnmarshaller();

			FileReader file = new FileReader("books.2.xml");
			BookList books = (BookList) um.unmarshal(file);

			file.close();

			BookManager bm = new BookManager(books.getBooks());
			List<String> new_book_authors = new ArrayList<>();
			new_book_authors.add("Michał");

		        Book ksiazka = new Book("1", "Laboratorium: Java i XML", new_book_authors, "111-222-333", 2018, 
                        "UCZELNIA WYDAWNICZA", 1);

		bm.addBook(ksiazka);

		List<Book> bs = bm.searchByTitle("Java");

		System.out.println(bs);
		BookList bls = new BookList();

		bls.setBooks(bs);

		System.out.println(bls);

		Marshaller m = context.createMarshaller();
		FileWriter fiW = new FileWriter("lab3.xml");

		m.marshal(bls, fiW);

		fiW.close();

	}
	catch (Exception ex) {
			ex.printStackTrace();
	}

 public Book(String id, String title, List<String> authors, String isbn, Integer year, String publisher, Integer pages) {
        this.id = id;
        this.title = title;
        this.authors.addAll(authors);
        this.isbn = isbn;
        this.year = year;
        this.publisher = publisher;
        this.pages = pages;
    }

this.authors.addAll(authors);

		Book ksiazka = new Book("1", "Laboratorium: Java i XML", new_book_authors, "111-222-333", 2018, "UCZELNIA WYDAWNICZA", 1);

@XmlRootElement(name = "books")
class BookList {
	
	private List<Book> books;

	@XmlElementWrapper
	@XmlElement(name = "book")
	public List<Book> getBooks() {
		return books;
	}

        this.authors.addAll(authors);

List<String> authors;

 List<String> authors=new ArrayList<>();

List<String> authors;

this.authors.addAll(authors);

    List<String> authors=new ArrayList<>();

java.lang.NullPointerException
	at books.BookManager.addBook(BookManager.java:47)




**KOD:**

public Book addBook(Book b) {
		if (b.getTitle().isEmpty() || b.getAuthors().isEmpty()) {
			throw new RuntimeException("nie ma tytułu");
		} else {
			books.add(b);
		}
		return b;
	}

		Book ksiazka = new Book("1", "Laboratorium: Java i XML", new_book_authors, "111-222-333", 2018, "UCZELNIA WYDAWNICZA", 1);