fix typing

Author: davidism

src/flask/app.py

@@ -1411,8 +1411,8 @@ def process_response(self, ctx: AppContext, response: Response) -> Response:
                 for func in reversed(self.after_request_funcs[name]):
                     response = self.ensure_sync(func)(response)
 
-        if not self.session_interface.is_null_session(ctx._session):
-            self.session_interface.save_session(self, ctx._session, response)
+        if not self.session_interface.is_null_session(ctx._get_session()):
+            self.session_interface.save_session(self, ctx._get_session(), response)
 
         return response