QnaMaker QuickStart Code: generate_answer: ErrorResponseException: (BadArgument) Invalid input. See details.

> ---------------------------------------------------------------

### This issue is for a: (mark with an `x`)
```
- [x] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
>
after creating knowledgebase in qnamaker, using the sample code under the preview sdk:
```
def download_kb(client, kb_id):
print("Downloading knowledge base...")
kb_data = client.knowledgebase.download(kb_id=kb_id, environment="Prod")
print("Downloaded knowledge base. It has {} QnAs.".format(len(kb_data.qna_documents)))
download_kb (client=client, kb_id=kb_id)
```
Downloading knowledge base...
Downloaded knowledge base. It has 9 QnAs.

but when i try generate answer, it gives an error message:
```
def generate_answer(client, kb_id):
print ("Querying knowledge base...")

listSearchResults = client.knowledgebase.generate_answer(kb_id, QueryDTO(question = "How do I manage my knowledgebase?"))

for i in listSearchResults.answers:
print(f"Answer ID: {i.id}.")
print(f"Answer: {i.answer}.")
print(f"Answer score: {i.score}.")

generate_answer(client=client, kb_id=kb_id)
```
### Any log messages given by the failure
```
ErrorResponseException: (BadArgument) Invalid input. See details.
---------------------------------------------------------------------------
ErrorResponseException Traceback (most recent call last)
~\AppData\Local\Temp/ipykernel_15788/4283219011.py in
9 print(f"Answer score: {i.score}.")
10
---> 11 generate_answer(client=client, kb_id=kb_id)

~\AppData\Local\Temp/ipykernel_15788/4283219011.py in generate_answer(client, kb_id)
2 print ("Querying knowledge base...")
3
----> 4 listSearchResults = client.knowledgebase.generate_answer(kb_id, QueryDTO(question = "How do I manage my knowledgebase?"))
5
6 for i in listSearchResults.answers:

~\AppData\Local\Programs\Python\Python39\lib\site-packages\azure\cognitiveservices\knowledge\qnamaker\operations\knowledgebase_operations.py in generate_answer(self, kb_id, generate_answer_payload, custom_headers, raw, **operation_config)
520
521 if response.status_code not in [200]:
--> 522 raise models.ErrorResponseException(self._deserialize, response)
523
524 deserialized = None

ErrorResponseException: (BadArgument) Invalid input. See details.
```

### Expected/desired behavior
>

### OS and Version?
> Windows 11 Version 21H2, Build 22000.120, python 3.9.6

### Versions
`client.api_version`

`'v5.0-preview.1'`

### Mention any other details that might be useful