Bad interaction between additionalProperties and $ref

Description

Compiling the following spec into a TypeScript-Fetch client results in invalid code.
Specifically, the generated code contains

export interface GenericMap extends null<String, number> {
}

which the TypeScript compiler rejects.

Swagger-codegen version

I tested it on swagger-codegen 2.2.1 (latest stable), 2.3.0 (7aebcfa) and master (d11d0f8).

Swagger declaration file content or url

...
definitions:
  GenericMap:
    additionalProperties:
      type: integer
  Container:
    type: object
    properties:
      works:
        type: object
        additionalProperties:
          type: string
      breaks:
        $ref: "#/definitions/GenericMap"

Command line used for generation

swagger-codegen generate -i test.yaml -l typescript-fetch

Steps to reproduce

Compile a swagger API containing the definitions mentioned above into a typescript-fetch client.

Further data

This was originally reported as https://github.com/swagger-api/swagger-codegen/issues/4839
@wing328 suggested to report the issue here as it is likely related to some problems with the resolution of the reference.

Start with the supplied YAML definition and run swagger-codegen generate -i test.yaml -l typescript-fetch against the reported versions or master. Trace how the $ref to GenericMap is resolved alongside additionalProperties; done means the generated TypeScript-Fetch client no longer contains invalid extends null<String, number> code.