It seems like currently only application/json is supported as MIME specifier. Instead the builder should also accept specifiers with the pattern application/myapp+json as is the case for e.g. GitHub (application/vnd.github.mercy-preview+json)

Additionally when using an OpenAPI document like below the kiota builder throws a NullReferenceException instead of a diagnostic error.

swagger: '2.0'
info:
  version: 1.0.0
  title: Sample
  description: ''
host: hostname
schemes:
  - https
paths:
  /something:
    get:
      description: ''
      produces:
        - application/myapp+json
      responses:
        '400':
          description: Indicates failure.
          schema:
            type: string