Snip bug: "should have been overridden context"

[Kshitij Sachan]

I'm setting up a very simple snip class that is based on the example in the documentation  (https://docs.racket-lang.org/gui/editor-overview.html#%28part._snip-example%29 Section 5.4). 

I was able to generate a snip that is a circle that grows in size when the up arrow key is pressed. However, after running the program again for some reason I ran into this bug:

error while rendering snip "test-snip":

should have been overridden  context:

   (...higher-order.rkt:364:33 . #f)

   (.../more-scheme.rkt:261:28 . #f)

   (loop . #(struct:srcloc #<path:/Applications/Racket v7.2/share/pkgs/gui-lib/framework/private/text.rkt> 2940 6 114655 2217))

   (|do-insertion method in ports-mixin| . #(struct:srcloc #<path:/Applications/Racket v7.2/share/pkgs/gui-lib/framework/private/text.rkt> 2933 4 114420 2804))

   (#f . #(struct:srcloc #<path:/Applications/Racket v7.2/share/pkgs/gui-lib/framework/private/text.rkt> 2926 9 114164 107))

   (#f . #(struct:srcloc #<path:/Applications/Racket v7.2/share/pkgs/gui-lib/mred/private/wx/common/queue.rkt> 435 6 19067 1056))

   (#f . #(struct:srcloc #<path:/Applications/Racket v7.2/share/pkgs/gui-lib/mred/private/wx/common/queue.rkt> 486 32 21054 120))

   (#f . #(struct:srcloc #<path:/Applications/Racket v7.2/share/pkgs/gui-lib/mred/private/wx/common/queue.rkt> 634 3 26076 58))

Here is my code as reference:

;; snip-class

(define test-snip-class

  (make-object

   (class snip-class%

     (super-new)

     (send this set-classname "test-snip"))))

;; snip stuff

(define scroll-snip%

  (class snip%

    (inherit get-admin set-snipclass set-flags)

    (init-field [w 50] [h 50])

    

    (super-new)

    (set-snipclass test-snip-class)

    (send (get-the-snip-class-list) add test-snip-class)

    (set-flags (list 'handles-events))

    

    (define/override (get-extent dc x y width height descent space lspace rspace)

      (define (maybe-set-box! b v) (when b (set-box! b v)))

        (maybe-set-box! width w)

        (maybe-set-box! height h)

        (maybe-set-box! descent 1.0)

        (maybe-set-box! space 1.0)

        (maybe-set-box! lspace 1.0)

        (maybe-set-box! rspace 1.0))

    

    (define/override (on-char dc x y editorx editory event)

      (cond

        [(equal? (send event get-key-code) 'up)

         (define admin (get-admin))

         (set! w (+ 20 w))

         (set! h (+ 20 h))

         (when admin

           (send admin resized this #t))]))

    (define/override (draw dc x y left top right bottom dx dy draw-caret)

      (send dc set-brush "red" 'solid)

      (send dc set-pen "black" 3 'solid)

      (send dc draw-ellipse (+ x 1.0) (+ y 1.0) (- w 2) (- h 2)))

    (define/override (copy)

      (new scroll-snip%))

    ))

(new scroll-snip%)

How can I fix this?