Following Loop Agent document，raise KeyError: 'Context variable not found: `initial_topic`.'

Hi, I'm totally new to ADK. When I follow the [Loop Agent](https://google.github.io/adk-docs/agents/workflow-agents/loop-agents/#full-example-iterative-document-improvement) document, I got the error below:
```
KeyError: 'Context variable not found: `initial_topic`.'
```
Steps to reproduce:
1. `mkdir loop_agent`, prepare everything
2. pasting the code in the document to `agent.py`
3. ran `adk run loop_agent`
4. input any sentence
5. The error above raised

How can I fix it? Can anyone help us? thanks