Referenced model with array causes NullpointerException

My api spec:

swagger: '2.0'
info:
  title: Test API
  description: This is my awesome documentation
  version: 1.0.0
host: test.com

schemes:
  - https
produces:
  - application/json

paths:
  /sites:
    get:
      description: |
        Get some sites
      responses:
        '200':
          description: Success
          schema:
            $ref: '#/definitions/sites'
definitions:
  sites:
    $ref: './schemas/sitesGETresponse.schema.json'

sitesGETresponse.schema.json:

{
    "id": "sitesGETresponse.schema.json#",
    "$schema": "http://json-schema.org/draft-04/schema#",
    "title": "Site list",
    "description": "Response to site list request",
    "type": "array",
    "items": {
        "type": "string"
    }
}

Created an error:

> java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -i ../path/to/my/swagger/api.yaml -o samples/client/test/android-java -l android -v
[main] INFO io.swagger.codegen.config.CodegenConfigurator -
VERBOSE MODE: ON. Additional debug options are injected
 - [debugSwagger] prints the swagger specification as interpreted by the codegen
 - [debugModels] prints models passed to the template engine
 - [debugOperations] prints operations passed to the template engine
 - [debugSupportingFiles] prints additional data passed to the template engine
reading from ../path/to/my/swagger/api.yaml
Exception in thread "main" java.lang.NullPointerException
    at io.swagger.parser.processors.ExternalRefProcessor.processRefToExternalDefinition(ExternalRefProcessor.java:63)
    at io.swagger.parser.processors.ModelProcessor.processRefModel(ModelProcessor.java:92)
    at io.swagger.parser.processors.ModelProcessor.processModel(ModelProcessor.java:33)
    at io.swagger.parser.processors.DefinitionsProcessor.processDefinitions(DefinitionsProcessor.java:40)
    at io.swagger.parser.SwaggerResolver.resolve(SwaggerResolver.java:42)
    at io.swagger.parser.SwaggerParser.read(SwaggerParser.java:64)
    at io.swagger.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:317)
    at io.swagger.codegen.cmd.Generate.run(Generate.java:186)
    at io.swagger.codegen.SwaggerCodegen.main(SwaggerCodegen.java:35)

Running swagger codegen built from source, master checkout

Reproduce the failure with the Swagger Codegen CLI command and the API YAML and external schema shown in the issue. Start at io.swagger.parser.processors.ExternalRefProcessor.java:63, then trace the call through ModelProcessor while handling the array definition. Done means this input no longer throws a NullPointerException during generation.