[BUG][TYPESCRIPT] Bug generating type declaration for value type List<SomeType> within a Map.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

Error resolving $ref pointer "https://apitools.dev/swagger-parser/online/#/definitions/List".
Token "List" does not exist.

Unable to generate typescript client due to the following Error:
model/NotificationEvents.ts:16:40 - error TS2694: Namespace '../dist/otr-ts-client/model/models"' has no exported member 'List'.

The result type is.

export interface NotificationEvents {
    "events"?: { [key: string]: models.List; };
    "totalUnreadCount"?: number;
}

openapi-generator version

openapi-generator-cli 4.3.1
commit : 003165c
built : 2020-05-06T09:38:39Z

OpenAPI declaration file content or url

NotificationEvents":{"properties":{"events":{"type":"object","additionalProperties":{"$ref":"#/definitions/List"}},"totalUnreadCount":{"type":"integer","format":"int64"}}}

Command line used for generation

openapi-generator generate -i api/otr-service.json -g typescript-angularjs -o dist/otr-ts-client"

Steps to reproduce

After adding @types/angular as a dependency, I just ran the command below

tsc index.ts

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/2336

Run the supplied openapi-generator command with api/otr-service.json and inspect the generated model/NotificationEvents.ts output. Trace how the additionalProperties $ref for List is resolved in the typescript-angularjs generator. Done means the generated declaration no longer references a nonexistent models.List and the generated TypeScript passes tsc.