Exception while running generate on validated format in multiple languages

Description

While trying to generate code for a valid swagger definition I get the following exception:

[main] INFO io.swagger.parser.Swagger20Parser - reading from /Users/patrick/Coginitive/Face/FaceSwagger.json
[main] INFO io.swagger.codegen.ignore.CodegenIgnoreProcessor - No .swagger-codegen-ignore file found.
Exception in thread "main" java.lang.RuntimeException: Could not process operation:
Tag: Tag {
name: default
description: null
externalDocs: null
extensions:{}}
Operation: 563879b61984550f30395236
Resource: post /detect
Definitions: {}
Exception: null
at io.swagger.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:932)
at io.swagger.codegen.DefaultGenerator.processPaths(DefaultGenerator.java:811)
at io.swagger.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:431)
at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:746)
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.fromParameter(DefaultCodegen.java:2585)
at io.swagger.codegen.DefaultCodegen.fromOperation(DefaultCodegen.java:2213)
at io.swagger.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:881)
... 5 more

I get the same exception whether I try to generate code for Swift 4 or C#, I have not tried any other language.

Swagger-codegen version

Using Swagger 2.3.1 on a Mac installed via Homebrew.

Swagger declaration file content or url

Using the swagger API definition for the following API:
https://westus.dev.cognitive.microsoft.com/docs/services/563879b61984550e40cbbe8d/operations/563879b61984550f30395236

I did validate the definition.

Command line used for generation

swagger-codegen generate -i -l csharp -o

swagger-codegen generate -i -l swift4 -o

Steps to reproduce

Execute the command above against the supplied definition.

Related issues/PRs

Suggest a fix/enhancement

Start by running the reported swagger-codegen generate command against the linked Cognitive Services definition for csharp and swift4. Trace the failure through SwaggerCodegen.main, Generate.run, DefaultGenerator.java, and DefaultCodegen.fromParameter. Done means both generation commands process the valid definition without the reported NullPointerException.