Getting NPE when generating a client from an OpenAPI spec with the JSON path reference

[TharmiganK]

Description

$Subject

Steps to Reproduce

Try to generate the client for the following spec:

openapi: 3.0.0
info:
  title: Albums API
  version: 1.0.0
  description: A simple API to get a list of albums
paths:
  /albums:
    get:
      summary: Get a list of albums
      responses:
        '200':
          description: A list of albums
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/AlbumArray'
  /albums/{id}:
    get:
      summary: Get an album by ID
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: integer
      responses:
        '200':
          description: An album
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/AlbumArray/items'
components:
  schemas:
    AlbumArray:
      type: array
      items:
        type: object
        properties:
          id:
            type: integer
            example: 1
          title:
            type: string
            example: Album Title
          artist:
            type: string
            example: Artist Name
          release_date:
            type: string
            format: date
            example: '2023-01-01'

$ bal openapi -i openapi.yaml --mode client --single-file
ballerina: Oh no, something really went wrong. Bad. Sad.

We appreciate it if you can report the code that broke Ballerina in
https://github.com/ballerina-platform/ballerina-lang/issues with the
log you get below and your sample code.

We thank you for helping make us better.

[2024-10-30 11:37:34,774] SEVERE {b7a.log.crash} - Cannot invoke "io.swagger.v3.oas.models.media.Schema.get$ref()" because "schemaValue" is null 
java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.media.Schema.get$ref()" because "schemaValue" is null
        at io.ballerina.openapi.core.generators.type.TypeGeneratorUtils.getTypeGenerator(TypeGeneratorUtils.java:67)
        at io.ballerina.openapi.core.generators.type.BallerinaTypesGenerator.generateTypeDescriptorNodeForOASSchema(BallerinaTypesGenerator.java:122)
        at io.ballerina.openapi.core.generators.type.BallerinaTypesGenerator.generateTypeDescriptorNodeForOASSchema(BallerinaTypesGenerator.java:88)
        at io.ballerina.openapi.core.generators.common.TypeHandler.getTypeNodeFromOASSchema(TypeHandler.java:196)
        at io.ballerina.openapi.core.generators.common.TypeHandler.getTypeNodeFromOASSchema(TypeHandler.java:191)
        at io.ballerina.openapi.core.generators.client.FunctionReturnTypeGeneratorImp.getDataType(FunctionReturnTypeGeneratorImp.java:188)
        at io.ballerina.openapi.core.generators.client.FunctionReturnTypeGeneratorImp.populateReturnType(FunctionReturnTypeGeneratorImp.java:135)
        at io.ballerina.openapi.core.generators.client.FunctionReturnTypeGeneratorImp.getReturnTypeInfo(FunctionReturnTypeGeneratorImp.java:113)
        at io.ballerina.openapi.core.generators.client.FunctionReturnTypeGeneratorImp.getReturnType(FunctionReturnTypeGeneratorImp.java:81)
        at io.ballerina.openapi.core.generators.client.ResourceFunctionSignatureGenerator.generateFunctionSignature(ResourceFunctionSignatureGenerator.java:86)
        at io.ballerina.openapi.core.generators.client.ResourceFunctionGenerator.generateFunction(ResourceFunctionGenerator.java:88)
        at io.ballerina.openapi.core.generators.client.BallerinaClientGenerator.addResourceFunction(BallerinaClientGenerator.java:653)
        at io.ballerina.openapi.core.generators.client.BallerinaClientGenerator.createResourceFunctions(BallerinaClientGenerator.java:643)
        at io.ballerina.openapi.core.generators.client.BallerinaClientGenerator.getClassDefinitionNode(BallerinaClientGenerator.java:260)
        at io.ballerina.openapi.core.generators.client.BallerinaClientGenerator.getModuleMemberDeclarationNodes(BallerinaClientGenerator.java:216)
        at io.ballerina.openapi.core.generators.client.BallerinaClientGenerator.getSyntaxTree(BallerinaClientGenerator.java:194)
        at io.ballerina.openapi.core.generators.client.BallerinaClientGenerator.generateSyntaxTree(BallerinaClientGenerator.java:188)
        at io.ballerina.openapi.cmd.BallerinaCodeGenerator.generateClientFiles(BallerinaCodeGenerator.java:428)
        at io.ballerina.openapi.cmd.BallerinaCodeGenerator.generateClient(BallerinaCodeGenerator.java:257)
        at io.ballerina.openapi.cmd.OpenApiCmd.generatesClientFile(OpenApiCmd.java:445)
        at io.ballerina.openapi.cmd.OpenApiCmd.openApiToBallerina(OpenApiCmd.java:368)
        at io.ballerina.openapi.cmd.OpenApiCmd.execute(OpenApiCmd.java:267)
        at java.base/java.util.Optional.ifPresent(Optional.java:178)
        at io.ballerina.cli.launcher.Main.main(Main.java:59)

Version

Ballerina SwanLake Update 10(2201.10.x)

Environment Details (with versions)

No response

[TharmiganK]

This is discovered from the smart sheet connector development. The original spec can be found in here: https://gist.github.com/TharmiganK/cab050e07357bf79bf745f3cdacdba11

The issue is with all the json path in the reference such as properties, allOf etc. Currently we only support basic references