Fixed #26917 -- Fixed crash in disabled ModelChoiceFields.

Partially reverted refs #25532 to fix a regression in Django 1.10.
This reintroduces a crash for disabled forms.JSONField (refs #26949),
however, that issue is also present on Django 1.9.

Thanks Ryan Schave for the test.

tests/model_forms/tests.py

@@ -1519,6 +1519,21 @@ class ModelChoiceFieldTests(TestCase):
             '<label><input name="foo" type="radio" value="" /> ---------</label>'
         )
 
+    def test_disabled_modelchoicefield(self):
+        class ModelChoiceForm(forms.ModelForm):
+            author = forms.ModelChoiceField(Author.objects.all(), disabled=True)
+
+            class Meta:
+                model = Book
+                fields = ['author']
+
+        book = Book.objects.create(author=Writer.objects.create(name='Test writer'))
+        form = ModelChoiceForm({}, instance=book)
+        self.assertEqual(
+            form.errors['author'],
+            ['Select a valid choice. That choice is not one of the available choices.']
+        )
+
     def test_modelchoicefield_iterator(self):
         """
         Iterator defaults to ModelChoiceIterator and can be overridden with