Add missing null=True and imports to modeladmin example code

docs/reference/contrib/modeladmin/index.rst

@@ -80,11 +80,16 @@ to create, view, and edit ``Book`` entries.
 
 .. code-block:: python
 
+    from django.db import models
+    from wagtail.admin.edit_handlers import FieldPanel
+    from wagtail.images.edit_handlers import ImageChooserPanel
+
     class Book(models.Model):
         title = models.CharField(max_length=255)
         author = models.CharField(max_length=255)
         cover_photo = models.ForeignKey(
             'wagtailimages.Image',
+            null=True, blank=True,
             on_delete=models.SET_NULL,
             related_name='+'
         )