[1.6.x] Fixed #16433 -- Fixed a help_text/read only field interaction that caused an admin crash.

Thanks chris at cogdon.org for the report and admackin for the patch.

Backport of af953c45cc from master

tests/admin_views/tests.py

@@ -49,7 +49,7 @@ from .models import (Article, BarAccount, CustomArticle, EmptyModel, FooAccount,
     OtherStory, ComplexSortedPerson, PluggableSearchPerson, Parent, Child, AdminOrderedField,
     AdminOrderedModelMethod, AdminOrderedAdminMethod, AdminOrderedCallable,
     Report, MainPrepopulated, RelatedPrepopulated, UnorderedObject,
-    Simple, UndeletableObject, Choice, ShortMessage, Telegram)
+    Simple, UndeletableObject, Choice, ShortMessage, Telegram, Pizza, Topping)
 from .admin import site, site2
 
 
@@ -3559,6 +3559,17 @@ class ReadonlyTest(TestCase):
         self.assertContains(response, '<p>No opinion</p>', html=True)
         self.assertNotContains(response, '<p>(None)</p>')
 
+    def test_readonly_backwards_ref(self):
+        """
+        Regression test for #16433 - backwards references for related objects
+        broke if the related field is read-only due to the help_text attribute
+        """
+        topping = Topping.objects.create(name='Salami')
+        pizza = Pizza.objects.create(name='Americano')
+        pizza.toppings.add(topping)
+        response = self.client.get('/test_admin/admin/admin_views/topping/add/')
+        self.assertEqual(response.status_code, 200)
+
 
 @override_settings(PASSWORD_HASHERS=('django.contrib.auth.hashers.SHA1PasswordHasher',))
 class RawIdFieldsTest(TestCase):