Code Gen API - Unable to build target: null

Description

When sending a request to https://generator.swagger.io/api/gen/clients/html the service returns error 400 with the foolowing response:
{ "code": 1, "type": "error", "message": "Unable to build target: null" }

All other languages are fine, html is the only one returning this error.

Swagger-codegen version

Sent to the api, using https://generator.swagger.io/api/gen/clients/html

Swagger declaration file content or url

https://gist.github.com/taroquu/2e812316647ea4e28eb9624559361b67

Command line used for generation

Generated by sending to the API.

Steps to reproduce

To https://generator.swagger.io/api/gen/clients/html send the following post request:

{ "spec": {}, "options": { "additionalProp1": "string", "additionalProp2": "string", "additionalProp3": "string" }, "swaggerUrl": "http://petstore.swagger.io/v2/swagger.json", "authorizationValue": { "value": "string", "type": "string", "keyName": "string" }, "securityDefinition": { "type": "string", "description": "string" } }

The value for spec can be filled in with the contents from the above gist.

Related issues/PRs

This is posibly similar to an older issue #1753

Start by reproducing the POST request against https://generator.swagger.io/api/gen/clients/html with the Swagger declaration from the linked gist, then compare its response with the other client-generation endpoints. Done means the HTML target builds successfully and the API no longer returns "Unable to build target: null" for the supplied specification.