[Python] not generating if query parameteres contain object

Description

I cannot generate python server (flask) or client if yaml contains query parameters based on an object.

Error thrown:

Exception in thread "main" java.lang.RuntimeException: Could not process operation:
  Tag: class Tag {
    name: default
    description: null
    externalDocs: null
}
[..]
  Exception: null
        at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:1039)
        at org.openapitools.codegen.DefaultGenerator.processPaths(DefaultGenerator.java:930)
        at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:511)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:891)
        at org.openapitools.codegen.cmd.Generate.run(Generate.java:355)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:62)
Caused by: java.lang.NullPointerException
        at org.openapitools.codegen.DefaultCodegen.getParameterFromRef(DefaultCodegen.java:4122)
        at org.openapitools.codegen.DefaultCodegen.fromOperation(DefaultCodegen.java:2472)
        at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:1007)
        ... 5 more

openapi-generator version

3.3.3
openapi-generator-cli-3.3.4-20181130.143646-28.jar
openapi-generator-cli-3.4.0-20181108.104734-7.jar
openapi-generator-cli-4.0.0-20181129.104836-46.jar

with 3.3.1 generating didn't throw any errors, but I had to adjust the code a little bit

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: Test    
  version: 0.1.0
  title: "Test API"

paths:
  /objects:
    get:
      summary: "Get all available objects"
      operationId: getObjects
      parameters:
      - in: "query"
        name: "filterTime"
        description: "Time based filter"
        required: false
        schema:
          type: "object"
          properties:
            begin:
              type: "string"
              format: "date-time"
            end:
              type: "string"
              format: "date-time"
          minProperties: 1
        style: "form"
        explode: true
      responses:
        200:
          description: "List of available objects"
          content:
            application/json:
              schema:
                type: "array"
                items:
                  allOf:
                  - $ref: '#/components/schemas/AnotherObject'
        default:
          description: "standard error"

components:
  schemas:
    TestObject:
      type: "object"
      properties:
        name:
          type: "string"
          example: "Testname"
          
    AnotherObject:
      allOf:
        - $ref: '#/components/schemas/TestObject'
        - type: "object"
          properties:
            anotherproperty:
              type: "string"

Command line used for generation

java -jar openapi-generator-cli.jar generate -i testapispec.yaml -g python-flask -o out/

Steps to reproduce

use mentioned yaml and command line

Related issues/PRs

I didn't find any related issue

Suggest a fix/enhancement

Objects in query parameters should be accepted as it is a valid spec according to editor.swagger.io

Start by reproducing the failure with the supplied YAML and the python-flask generation command. Trace DefaultGenerator.processOperation and DefaultCodegen.getParameterFromRef, where the stack trace reports the NullPointerException. Done means the object query parameter is accepted according to the OpenAPI declaration and generation completes without the exception.