Providing allowed models in the config breaks settings UI

[DzmitrySudnik]

Description

After one of recent upgrades (either 2.16 or 2.15, I suspect this PR cf20800) config UI stopped working for me. Seems it's related to the allowed models I have configured. Removing allowed_models from the config solves the issue, however I need to control the allowed models.

Reproduce

1. Create a config file with allowed_models list
2. Restart jupyterlab
3. Go to Jupyter AI settings
4. You will see "Error: Unhandled error"

Expected behavior

No error on UI

Context

example of the config file:

{
  "AiExtension": {
    "allowed_models": [
      "bedrock-chat:anthropic.claude-v2",
      "bedrock-chat:anthropic.claude-v2:1",
      "bedrock-chat:anthropic.claude-3-sonnet-20240229-v1:0",
      "bedrock:amazon.titan-embed-text-v1"
    ]
 }
}

error in jupyterlab:

  File "/opt/conda/lib/python3.11/site-packages/tornado/web.py", line 1788, in _execute
    result = method(*self.path_args, **self.path_kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/tornado/web.py", line 3301, in wrapper
    return method(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/jupyter_ai/handlers.py", line 353, in get
    providers = self._filter_blocked_models(providers)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/jupyter_ai/handlers.py", line 293, in _filter_blocked_models
    provider.chat_models = list(filter(filter_predicate, provider.chat_models))
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: 'NoneType' object is not iterable```

[3coins]

@DzmitrySudnik
Thanks for reporting this issue. I can reproduce this issue when either of allowed_models or blocked_models is set.