Non-nullable property 'userName' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.

Non-nullable property 'password' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.

Non-nullable property 'isbn' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.

Non-nullable property 'title' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.

Non-nullable property 'subTitle' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.

Non-nullable property 'author' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.

Non-nullable property 'publisher' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.

Non-nullable property 'description' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.

Non-nullable property 'website' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.