Avoided leaking state on exceptions in populate_models().

django/apps/registry.py

@@ -125,10 +125,11 @@ class Apps(object):
                     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()