Grails - Cannot invoke method getAt() on null object - exception

Question

I am getting an error when trying to append a book to a list of classes list of books. Any ideas as to why or what I could do to fix it?
//For each book in a class...
for (int k = 0; k < rows.size(); k++) {
Book book = new Book()
//Assign the values to a new book object
book.id = rows[k].getProperty("ISBN")
book.title = rows[k].getProperty("title")
book.author = rows[k].getProperty("author")
book.required = rows[k].getProperty("required_optional")
book.purchaseType = rows[k].getProperty("rental_purchase")
//book.purchasePrice = rows[k].getProperty("purchase_price")
//book.rentalPrice = rows[k].getProperty("rental_fee")
//Append the book to the books list object in the particular class
classes[i].books[k + 1] << book
}

Answer 1

Instead of accessing by [] use getAt, then the ? Operator will work:
classes?.getAt(i)?.books?.getAt(k+1) << book
or
classes?.getAt(i)?.books[k+1] << book

Answer 2

rows.each { row ->
Book book = new Book(...)
classes[i].books << book
}

Answer 3

without the full code it's hard to know what happens. try this code, it will show you which value is null:
// first check if classes[i] is null, add error handling if it is
if (classes[i]!=null) {
//For each book in a class...
for (int k = 0; k < rows.size(); k++) {
def books = classes[i].books[k + 1]
if (books!=null) {
// you can set values directly here
def row = rows[k]
Book book = new Book(
id: k.getProperty("ISBN"),
title: k.getProperty("title")
// ...
)
//Append the book to the books list object in the particular class
books << book
} else {
println "books is null, do something clever"
}
}
} else {
println "classes[i] is null, do something clever"
}
I didn't test it, and you can probably make it shorter with some groovy goodness. this is just to get you started

Answer 4

I am not sure what 'classes' is, but if its a list containing 'Book' you can just do
classes << book
OR
classes.add(book)