Avoided leaking state on exceptions in populate_models().

                    self._postponed.append(app_config)

            if outermost:

                try:

                    for app_config in self._postponed:

                        all_models = self.all_models[app_config.label]

                        app_config.import_models(all_models)

                finally:

                    del self._postponed

                self.clear_cache()