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

#### Bug Report Checklist

- [x] Have you provided a full/minimal spec to reproduce the issue?
- [x] Have you validated the input using an OpenAPI validator ([example](https://apidevtools.org/swagger-parser/online/))?
- [x] Have you [tested with the latest master](https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-to-test-with-the-latest-master-of-openapi-generator) to confirm the issue still exists?
- [x] Have you searched for related issues/PRs?
- [ ] What's the actual output vs expected output?
- [ ] [Optional] Sponsorship to speed up the bug fix or feature request ([example](https://github.com/OpenAPITools/openapi-generator/issues/6178))



##### 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

```yaml
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



##### Related issues/PRs



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Uh oh!

[BUG] [Golang][Client] java.lang.NullPointerException: Cannot read field "name" because "codegenModel" is null #12357

Bug Report Checklist

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Uh oh!

[BUG] [Golang][Client] java.lang.NullPointerException: Cannot read field "name" because "codegenModel" is null #12357

Description

Bug Report Checklist

Description

openapi-generator version

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

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