[BUG][Dart] the generate code has error by using additionalProperties

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
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• What's the actual output vs expected output?
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Description

I use additionalProperties in my yaml file and generate dart code but get the error.

openapi-generator version

openapi-generator-cli-6.6.0.jar

OpenAPI declaration file content or url

Here is my yaml setting

    roles:
      type: object
      additionalProperties:
        type: array
        items:
          type: string

The generate code

roles: json[r'roles'] == null
      ? const {}
        : mapCastOfType<String, List>(json, r'roles'),

The error

The argument type 'Map<String, List<dynamic>>?' can't be assigned to the parameter type 'Map<String, List<String>>'.

How to fix the error?
Thanks.

Reproduce the issue with the provided OpenAPI YAML and openapi-generator-cli 6.6.0 using the Dart generator. Start by tracing Dart handling of additionalProperties and inspect the generated roles assignment; done means the generated code accepts the declared Map<String, List> type without the reported type error.