[BUG][spring] definition of `type: object`, `items: # ...` passes as valid but isn't and causes compiler errors

Bug Report Checklist

Description

The spec below is invalid (an object has no items property), yet doesn't cause a validation error in either the Swagger Editor or when parsed by org.openapitools:openapi-generator-maven-plugin:5.3.0:

When used in a maven "build pipeline" with the spring generator, it results in a java.lang.AssertionError (apparently thrown by the Java compiler).

When using references to remote files (splitting the specification into multiple files) one can observe the following warning output (depending on setup) being logged:
https://github.com/OpenAPITools/openapi-generator/blob/0bb4f186ec7e776efbe370be89798ed2f659c9ff/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultGenerator.java#L458

openapi-generator version

5.3.0 (via maven plugin; cf. above)

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: object with items
  version: 1.0.0
servers:
- url: /
tags:
- name: pet
paths:
  /cat:
    get:
      tags:
      - pet
      operationId: getCats
      responses:
        200:
          description: Cats
          content:
            application/json:
              schema:
                type: object # should be array
                items:
                  type: string

Generation Details

• org.openapitools:openapi-generator-maven-plugin:5.3.0
• spring generator

Steps to reproduce

Paste the invalid YAML into the Swagger editor. It doesn't display an error.

Using a build pipeline, have the plugin generate model and api files. You should get the Java error.

Related issues/PRs

n/a

Suggest a fix

Clear validation error message

Start with the validation path used by the spring generator and inspect the referenced DefaultGenerator.java location around line 458. Reproduce the YAML with the Maven plugin and confirm how an object schema with items is handled. Done means the invalid schema produces a clear validation error before generation instead of a compiler failure.