[CSharp] Could not process model bug

Description

I use simple YAML (JSON tested too) and have a problem with clients generation.

The error:

[main] INFO io.swagger.parser.Swagger20Parser - reading from ./swaggers/build.yaml
[main] INFO io.swagger.codegen.languages.CSharpClientCodegen - Generating code for .NET Framework v4.5
Exception in thread "main" java.lang.RuntimeException: Could not process model 'BranchConfiguration'.Please make sure that your schema is correct!
        at io.swagger.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:328)
        at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:697)
        at io.swagger.codegen.cmd.Generate.run(Generate.java:285)
        at io.swagger.codegen.SwaggerCodegen.main(SwaggerCodegen.java:35)
Caused by: java.lang.NullPointerException
        at io.swagger.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:1284)
        at io.swagger.codegen.languages.CSharpClientCodegen.fromModel(CSharpClientCodegen.java:430)
        at io.swagger.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:1266)
        at io.swagger.codegen.languages.CSharpClientCodegen.fromModel(CSharpClientCodegen.java:433)
        at io.swagger.codegen.DefaultGenerator.processModels(DefaultGenerator.java:939)
        at io.swagger.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:323)
        ... 3 more

Swagger-codegen version

Latest, 2.2.3 version

Swagger declaration file

swagger: '2.0'
host: api.appcenter.ms
basePath: /
schemes:
  - https
consumes:
  - application/json
produces:
  - application/json
paths:
  '/v0.1/apps/{owner_name}/{app_name}/branches/{branch}/config':
    get:
      responses:
        '200':
          schema:
            $ref: '#/definitions/BranchConfiguration'
      operationId: branchConfigurations_get
      tags:
        - build
definitions:
  BranchConfiguration:
    properties:
      trigger:
        type: string
        enum:
          - continuous
          - manual
    additionalProperties:
      $ref: '#/definitions/BranchProperties'
  BranchProperties:
    properties:
      enabled:
        type: boolean

Command line used for generation

swagger-codegen generate -i ./swagger.yaml -l csharp -o ./clients/swagger-codegen-csharp

Reproduce the failure with the supplied Swagger YAML and the swagger-codegen generate command, then trace the stack from DefaultCodegen.fromModel through CSharpClientCodegen.fromModel. Use the BranchConfiguration and BranchProperties definitions as the focused case; done means C# model generation completes without the reported NullPointerException.