[BUG] [Golang][Client] java.lang.NullPointerException: Cannot read field "name" because "codegenModel" is null

Created by: okhowang

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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?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

run generator got exception with yaml

Exception in thread "main" java.lang.RuntimeException: Could not generate api file for 'Default'
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:668)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:895)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:449)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.NullPointerException: Cannot read field "name" because "codegenModel" is null
	at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:660)
	at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:683)
	at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:639)
	at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:595)
	at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:693)
	at org.openapitools.codegen.languages.GoClientCodegen.constructExampleCode(GoClientCodegen.java:565)
	at org.openapitools.codegen.languages.GoClientCodegen.postProcessOperationsWithModels(GoClientCodegen.java:487)
	at org.openapitools.codegen.DefaultGenerator.processOperations(DefaultGenerator.java:1215)
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:572)
	... 4 more

Process finished with exit code 1

openapi-generator version

v5.4.0 master[02f16267]

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  version: 0.0.1
  title: example
paths:
  /:
    post:
      summary: demo
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Model'
      responses:
        200:
          description: OK
          content:
            application/json:
              schema:
                type: string

components:
  schemas:
    Item:
      oneOf:
        - type: string
        - type: integer
    Model:
      type: object
      required:
        - Items
      properties:
        Items:
          type: array
          minItems: 1
          items:
            $ref: '#/components/schemas/Item'

Generation Details

generate -g go -i test\openapi.yaml -o test\go

Steps to reproduce

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