Internal error when getting pen% and brush% color

[Bogdanp]

I happened to try this while looking into https://github.com/racket/gui/issues/307:

#lang racket/base

(require racket/class
         racket/draw)
(define p (new pen%))
(send p get-color)
(send p set-color "notacolor")
(send p get-color)

=>

(object:color% ...)
get-color: broke its own contract
  promised: (is-a?/c color%)
  produced: #f
  in: the range of
      the get-color method in
      brush%/c
  contract from:
      <pkgs>/draw-lib/racket/draw.rkt
  contract on: brush%
  blaming: <pkgs>/draw-lib/racket/draw.rkt
   (assuming the contract is correct)
  at: <pkgs>/draw-lib/racket/draw.rkt:85:19
  context...:
   /Users/bogdan/sandbox/racket/racket/collects/racket/contract/private/blame.rkt:346:0: raise-blame-error
   .../private/arrow-higher-order.rkt:375:33
   body of "/var/folders/11/9c7x7y096r9g35f9th6b93qc0000gn/T/tmp.ArY5q8ZT/draw.rkt"

The brush% class has the same issue.