You are tasked with writing an AutoGen app that creates two AI agents to collaborate on a user-specified task. One agent will be responsible for executing the task using search and open tools, while the other agent will manage and oversee the process. Follow these instructions to create the app:

1. Create two agents:
   a. ExecutorAgent: This agent will use search and open tools to complete the task.
   b. ManagerAgent: This agent will oversee the process and ensure steps are completed correctly.

2. For the ExecutorAgent, implement the following features:
   a. Use the ReAct (Reasoning and Acting) framework as described in https://microsoft.github.io/autogen/docs/topics/prompting-and-reasoning/react
   b. Implement reflection capabilities as outlined in https://microsoft.github.io/autogen/docs/topics/prompting-and-reasoning/reflection

3. For the ManagerAgent, implement the following features:
   a. Ability to review and critique the ExecutorAgent's work
   b. Capability to provide guidance and suggest improvements
   c. Final approval of completed steps

4. Implement a mechanism for saving results in a .txt file.

5. Use the following format for your AutoGen app code:

```python
import autogen

# Define the ExecutorAgent
executor_agent = autogen.AssistantAgent(
    name="ExecutorAgent",
    llm_config={
        "config_list": [{"model": "gpt-4"}],
        "temperature": 0,
    },
    system_message="""You are the ExecutorAgent responsible for completing tasks using search and open tools. 
    Use the ReAct framework for reasoning and acting, and implement reflection capabilities to improve your performance. 
    Your goal is to complete the given task efficiently and accurately."""
)

# Define the ManagerAgent
manager_agent = autogen.AssistantAgent(
    name="ManagerAgent",
    llm_config={
        "config_list": [{"model": "gpt-4"}],
        "temperature": 0,
    },
    system_message="""You are the ManagerAgent responsible for overseeing the ExecutorAgent's work. 
    Review and critique their progress, provide guidance, and ensure all steps are completed correctly. 
    Your goal is to manage the process and approve the final results."""
)

# Define the user proxy
user_proxy = autogen.UserProxyAgent(
    name="UserProxy",
    human_input_mode="TERMINATE",
    max_consecutive_auto_reply=10,
    is_termination_msg=lambda x: x.get("content", "").rstrip().endswith("TERMINATE"),
    code_execution_config={
        "work_dir": "coding",
        "use_docker": False,
    },
)

# Define the task
task = """<task>
{{USER_TASK}}
</task>"""

# Start the conversation
user_proxy.initiate_chat(
    manager_agent,
    message=f"""Please oversee the completion of the following task:
    {task}
    Work with the ExecutorAgent to complete this task efficiently and accurately. 
    Ensure all steps are properly executed and the final results are saved in a .txt file."""
)

# Save results to a file
with open("task_results.txt", "w") as f:
    f.write(user_proxy.last_message()["content"])

print("Task completed. Results saved in task_results.txt")
```

6. Replace the `{{USER_TASK}}` placeholder in the code with the actual task provided by the user.

7. Ensure that the app includes proper error handling and logging mechanisms.

8. Test the app thoroughly with various user tasks to ensure it functions correctly.

Remember to adapt the code as necessary based on the specific requirements of the user's task and any additional AutoGen features you may need to implement.