Javascript – Cannot read properties of null (reading 'name')

I am following The Odin’s project local library tutorial (Express app).
Here is my code (only relevant parts):

in routes/catalog.js I have this

router.get("/books", book_controller.book_list);

models/book.js

const mongoose = require('mongoose');

const Schema = mongoose.Schema;

const BookSchema = new Schema({
    title:{type:String,required:true},
    author:{type:Schema.Types.ObjectId,ref:"Author",required:true},
    summary:{type:String,required:true},
    isbn:{type:String,required:true},
    genre:[{type: Schema.Types.ObjectId,ref:"Genre"}],
});
BookSchema.virtual("url").get(function(){
    return `/catalog/book/${this._id}`
})

module.exports = mongoose.model("Book",BookSchema);

controllers/bookController.js

const Book = require("../models/book");
const Author = require("../models/author");
const Genre = require("../models/genre");
const BookInstance = require("../models/bookinstance");

const asyncHandler = require("express-async-handler");

exports.index = asyncHandler(async (req, res, next) => {
  // Get details of books, book instances, authors and genre counts (in parallel)
  const [
    numBooks,
    numBookInstances,
    numAvailableBookInstances,
    numAuthors,
    numGenres,
  ] = await Promise.all([
    Book.countDocuments({}).exec(),
    BookInstance.countDocuments({}).exec(),
    BookInstance.countDocuments({ status: "Available" }).exec(),
    Author.countDocuments({}).exec(),
    Genre.countDocuments({}).exec(),
  ]);

  res.render("index", {
    title: "Local Library Home",
    book_count: numBooks,
    book_instance_count: numBookInstances,
    book_instance_available_count: numAvailableBookInstances,
    author_count: numAuthors,
    genre_count: numGenres,
  });
});

exports.book_list = asyncHandler(async (req, res, next) => {
  const allBooks = await Book.find({}, "title author")
    .sort({ title: 1 })
    .populate("author")
    .exec();

  res.render("book_list", { title: "Book List", book_list: allBooks });
});

Now here is the file which causes problems: views/book_list.pug

extends layout

block content
  h1= title

  ul
    each book in book_list
      li
        a(href=book.url) #{book.title}
        |  (#{book.author.name})

    else
      li There are no books.

I have no idea why is that causing an error? that clearly should work and it works in the tutorial so I must have something wrong here, but for the love of god I can’t find a mistake.
The error appears when I go to the route /catalog/books

D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugviewsbook_list.pug:10 8| li 9| a(href=book.url) #{book.title} > 10| | (#{book.author.name}) 11| 12| else 13| li There are no books. Cannot read properties of null (reading 'name')
TypeError: D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugviewsbook_list.pug:10
    8|       li
    9|         a(href=book.url) #{book.title}
  > 10|         |  (#{book.author.name})
    11| 
    12|     else
    13|       li There are no books.

Cannot read properties of null (reading 'name')
    at eval (eval at wrap (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulespug-runtimewrap.js:6:10), <anonymous>:120:69)
    at eval (eval at wrap (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulespug-runtimewrap.js:6:10), <anonymous>:155:4)
    at template (eval at wrap (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulespug-runtimewrap.js:6:10), <anonymous>:157:155)
    at exports.renderFile (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulespuglibindex.js:418:38)
    at exports.renderFile (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulespuglibindex.js:408:21)
    at exports.__express [as engine] (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulespuglibindex.js:455:11)
    at View.render (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulesexpresslibview.js:135:8)
    at tryRender (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulesexpresslibapplication.js:640:10)
    at Function.render (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulesexpresslibapplication.js:592:3)
    at ServerResponse.render (D:THE ODIN PROJECTNodeJSCourseexpress-locallib-pugnode_modulesexpresslibresponse.js:1008:7)

I was going all by myself and just looking at the general idea. When I encountered this for the first time I thought it’s because I’m using EJS and the tutorial goes by Pug. So I quickly switched the files and view engine to pug, but it didn’t help at all. Then I thought that it might be compatibility problem so I completely cloned this project by I generating the project as pug natively and copying the rest of the files. The error still exists – still at the same time.
When I remove book_list.pug there is no error (obviously it is caused by the

 > 10|         |  (#{book.author.name})

line, however I have no idea why is that causing problems on my part, and works in the tutorial? I can’t find the solution to this either.