Editor not catching $ref error

System information

• OS: macOS 10.14
• Browser: Chrome
• Version: 75.0
• Swagger/OpenAPI version: OpenAPI 3.0
• Found in the online swagger editor
  https://editor.swagger.io/

Example yaml to reproduce the error

module:
    post:
      tags:
      - module
      summary: Create a module
      operationId: createModule
      security:
        - TokenAuth: []
      requestBody:
        required: true
        content:
          application/json:
            schema:
              oneOf:
                - $ref: '#/components/schemas/BaseModule'
                - $ref: '#/components/schemas/FullModule'
      responses:
        200:
          description: "Success: module created"
...
components:
  schemas:
    BaseModule:
      description: Basic information about a model
      type: object
      properties:
        moduleId:
          type: integer
          format: int64
        moduleName:
          type: string
        details:
          type: string
        numberOfWords:
          type: integer
          format: int32
          example: 10
        parent:
          type: integer
          format: int64
        child:
          type: integer
          format: int64
      required:
        - moduleName

    FullModule:
      allOf:
        - $ref: '#/components/schemas/InfoModule'
        - type: object
          properties:
            words:
              type: array
              items:
                $ref: '#/components/schemas/Dictionary'
            quiz:
              type: array
              items:
                $ref: '#/components/schemas/Quiz'
      required:
        - words

Expected behavior

• The code above is marked as valid by the online editor as it appears to cause errors with Codegen
• utilizing the Codegen functionality to build a python-flask server stub I get the following error

There was an error generating the servers python-flask code. Please ensure that the definition and all external refs (inc. domains) are valid.

• I tried to generate the code locally with swagger-codegen generate -i Documents/asltutor.ymal -l python-flask -o Desktop and got the following error:

java.lang.NullPointerException
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromProperty(DefaultCodegenConfig.java:1685)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromRequestBody(DefaultCodegenConfig.java:2589)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromOperation(DefaultCodegenConfig.java:2102)
	at io.swagger.codegen.v3.DefaultGenerator.processOperation(DefaultGenerator.java:901)
	at io.swagger.codegen.v3.DefaultGenerator.processPaths(DefaultGenerator.java:828)
	at io.swagger.codegen.v3.DefaultGenerator.generateApis(DefaultGenerator.java:462)
	at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:776)
	at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:342)
	at java.lang.Thread.run(Thread.java:748)
Exception in thread "Thread-1" java.lang.RuntimeException: Could not process operation:
  Tag: class Tag {
    name: module
    description: null
    externalDocs: null
}
  Operation: create_module
  Resource: post /modules
  Exception: null
	at io.swagger.codegen.v3.DefaultGenerator.processOperation(DefaultGenerator.java:926)
	at io.swagger.codegen.v3.DefaultGenerator.processPaths(DefaultGenerator.java:828)
	at io.swagger.codegen.v3.DefaultGenerator.generateApis(DefaultGenerator.java:462)
	at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:776)
	at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:342)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromProperty(DefaultCodegenConfig.java:1685)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromRequestBody(DefaultCodegenConfig.java:2589)
	at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromOperation(DefaultCodegenConfig.java:2102)
	at io.swagger.codegen.v3.DefaultGenerator.processOperation(DefaultGenerator.java:901)
	... 5 more

• The error appears to be coming from the following section:

        content:
          application/json:
            schema:
              oneOf:
                - $ref: '#/components/schemas/BaseModule'
                - $ref: '#/components/schemas/FullModule'

• change to the following and code can be generated without error:

        content:
          application/json:
            schema:
              $ref: '#/components/schemas/FullModule'

Additional thoughts

• I suspect the code generator is working as expected but the online editor is not catching the implementation error and states that the code is valid
• strangely the code-gen error does not occur when building a spring, inflector or aspnetcore server stub. No idea why.

Reproduce the supplied OpenAPI YAML in the online editor, then compare its validation with the python-flask generation failure. Start with the reported DefaultCodegenConfig.java and DefaultGenerator.java stack-trace locations; done means the editor and generator agree on the oneOf case and the failure is covered by a reproducible check.