Refs #15667 -- Fixed crash when indexing RadioFieldRenderer with ModelChoiceIterator.

Regression in 86573861a9

django/forms/widgets.py

@@ -693,7 +693,7 @@ class ChoiceFieldRenderer(object):
         self.choices = choices
 
     def __getitem__(self, idx):
-        choice = self.choices[idx]  # Let the IndexError propagate
+        choice = list(self.choices)[idx]  # Let the IndexError propagate
         return self.choice_input_class(self.name, self.value, self.attrs.copy(), choice, idx)
 
     def __str__(self):

tests/model_forms/tests.py

@@ -1593,6 +1593,13 @@ class ModelChoiceFieldTests(TestCase):
         with self.assertNumQueries(1):
             template.render(Context({'field': field}))
 
+    def test_modelchoicefield_index_renderer(self):
+        field = forms.ModelChoiceField(Category.objects.all(), widget=forms.RadioSelect)
+        self.assertEqual(
+            str(field.widget.get_renderer('foo', [])[0]),
+            '<label><input name="foo" type="radio" value="" /> ---------</label>'
+        )
+
     def test_modelchoicefield_iterator(self):
         """
         Iterator defaults to ModelChoiceIterator and can be overridden with