title: Attribute Error: 'NoneType' object has no attribute 'id'
body: <p>I have reached till chapter 4 of the Mozilla Django tutorial, but then I met with this error. I followed everything as it said but it is giving me this error when I tried to open the <strong><em>BookInstance</em></strong> model from the admin panel : </p>
  
  <blockquote>
    <p>AttributeError at /admin/catalog/bookinstance/ 'NoneType' object has
    no attribute 'id'</p>
  </blockquote>
  
  <p>Here is my code, <strong>models.py</strong>( I've highlighted the part where the error occurs) :</p>
  
  <pre><code>from django.db import models
  from django.core.urlresolvers import reverse
  
  class Book(models.Model):
      """
      Model representing a book (but not a specific copy of a book).
      """
      title = models.CharField(max_length=200)
      author = models.ForeignKey('Author', on_delete=models.SET_NULL, null=True)
        # Foreign Key used because book can only have one author, but authors can have multiple books
        # Author as a string rather than object because it hasn't been declared yet in file.
      summary = models.TextField(max_length=1000, help_text="Enter a brief description of the book")
      isbn = models.CharField('ISBN',max_length=13, help_text='13 Character &lt;a href="https://www.isbn-international.org/content/what-isbn"&gt;ISBN number&lt;/a&gt;')
      genre = models.ManyToManyField(Genre, help_text="Select a genre for this book")
        # ManyToManyField used because Subject can contain many books. Books can cover many subjects.
        # Subject declared as an object because it has already been defined.
  
      def display_genre(self):
          """
          Creates a string for the Genre. This is required to display genre in Admin.
          """
          return ', '.join([ genre.name for genre in self.genre.all()[:3] ])
          display_genre.short_description = 'Genre'
  
  
      def get_absolute_url(self):
          """
          Returns the url to access a particular book instance.
          """
          return reverse('book-detail', args=[str(self.id)])
  
      def __str__(self):
          """
          String for representing the Model object.
          """
          return self.title
  
  
  
  import uuid # Required for unique book instances
  
  class BookInstance(models.Model):
      """
      Model representing a specific copy of a book (i.e. that can be borrowed from the library).
      """
      id = models.UUIDField(primary_key=True, default=uuid.uuid4, help_text="Unique ID for this particular book across whole library")
      book = models.ForeignKey(Book, on_delete=models.SET_NULL, null=True)
      imprint = models.CharField(max_length=200)
      due_back = models.DateField(null=True, blank=True)
  
      LOAN_STATUS = (
          ('d', 'Maintenance'),
          ('o', 'On loan'),
          ('a', 'Available'),
          ('r', 'Reserved'),
      )
  
      status = models.CharField(max_length=1, choices=LOAN_STATUS, blank=True, default='d', help_text='Book availability')
  
      class Meta:
          ordering = ["due_back"]
  
  
      def __str__(self):
          """
          String for representing the Model object
          """
          ***return '%s (%s)' %(self.id,self.book.title)***
  </code></pre>
  
  <p>Any help would be appreciated!</p>
  
  <p>EDIT : HERE IS THE COMPLETE ERROR
  <img src="https://i.stack.imgur.com/UlUFK.png" alt="Click here to view the error"></p>
  
  <p>EDIT: fixed it</p>
  
  <p>I replaced it with </p>
  
  <pre><code>        return str('%s (%s)'% (self.id, self.book.title))
  </code></pre>