6-lang language is not compatible with new DrRacket's

[formalizm]

Describe the bug
After successful linking of the 6-lang language module via raco, opening world.rkt (with #lang txtadv) in DrRacket 8.8 gives the following DrRacket internal error in a GUI message box popup:

hash-ref: contract violation
  expected: hash?
  given: "\n\n"
  argument position: 1st
  other arguments...:
   'type
  context...:
   C:\Program Files\Racket\share\pkgs\gui-lib\framework\private\color.rkt:54:0: attribs->type
   C:\Program Files\Racket\share\pkgs\gui-lib\framework\private\color.rkt:380:4: continue-re-tokenize method in text-mixin
   C:\Program Files\Racket\share\pkgs\gui-lib\framework\private\color.rkt:619:4: colorer-driver method in text-mixin
   C:\Program Files\Racket\share\pkgs\gui-lib\framework\private\color.rkt:642:4: colorer-callback method in text-mixin
   C:\Program Files\Racket\share\pkgs\gui-lib\mred\private\wx\common\queue.rkt:435:6
   C:\Program Files\Racket\share\pkgs\gui-lib\mred\private\wx\common\queue.rkt:486:32
   C:\Program Files\Racket\share\pkgs\gui-lib\mred\private\wx\common\queue.rkt:634:3

and the same text repeated 3 times in an appearing terminal.

To Reproduce
Steps to reproduce the behavior:

1. Follow instructions in https://github.com/spdegabrielle/flatt-languages/blob/master/6-color/README.txt to link the language via raco.
2. Open https://github.com/spdegabrielle/flatt-languages/blob/master/6-color/world.rkt in DrRacket.

Expected behavior
Expected no error, of course.
However, it's not clear what should be the effect of this https://github.com/spdegabrielle/flatt-languages/tree/master/6-color/lang language on coloring. It does not produce errors in DrRacket 7.7 (opens world.rkt OK), but the colors are identical to colors before linking this language via raco (both in normal and 'check syntax' modes).

Desktop configuration:

• OS: Windows 10
• Racket: 8.8 [default, cs]

Additional remarks
Apparently, there were some changes in DrRacket's API between versions 7.7 and 8.8.