Collect serialziers to try when rendering the Browsable API

The first one should work, if it doesn't because of Issue #2918, then
the second one should work. If no collected serializer worked, raise
the exception generated by the last one.

rest_framework/renderers.py

@@ -473,23 +473,26 @@ class BrowsableAPIRenderer(BaseRenderer):
             ):
                 return
 
+            serializers = []
             if existing_serializer is not None:
-                serializer = existing_serializer
+                serializers.append(existing_serializer)
-            else:
+
             if has_serializer:
                 if method in ('PUT', 'PATCH'):
-                        serializer = view.get_serializer(instance=instance, **kwargs)
+                    serializers.append(view.get_serializer(instance=instance, **kwargs))
                 else:
-                        serializer = view.get_serializer(**kwargs)
+                    serializers.append(view.get_serializer(**kwargs))
             else:
                 # at this point we must have a serializer_class
                 if method in ('PUT', 'PATCH'):
-                        serializer = self._get_serializer(view.serializer_class, view,
+                    serializers.append(self._get_serializer(view.serializer_class, view,
-                                                          request, instance=instance, **kwargs)
+                                                            request, instance=instance, **kwargs))
                 else:
-                        serializer = self._get_serializer(view.serializer_class, view,
+                    serializers.append(self._get_serializer(view.serializer_class, view,
-                                                          request, **kwargs)
+                                                            request, **kwargs))
 
+            for serializer in serializers:
+                try:
                     if hasattr(serializer, 'initial_data'):
                         serializer.is_valid()
 
@@ -499,6 +502,11 @@ class BrowsableAPIRenderer(BaseRenderer):
                         self.accepted_media_type,
                         {'style': {'template_pack': 'rest_framework/horizontal'}}
                     )
+                except TypeError:
+                    pass
+            # If none of the rendering attempts succeeded, raise the TypeError generated by the last
+            # attempt.
+            raise
 
     def get_raw_data_form(self, data, view, method, request):
         """