DrRacket Internal Error around coroutine-value

The error looks like it's coming from line 459 of drracket/private/syncheck/blueboxes-gui.rkt:

(define tag+rng (coroutine-value update-the-strs-coroutine))

In the update-the-strs method. I don't know how this is possible since lines 447-449 should have set the update-the-strs-coroutine variable to be a valid (non-false) coroutine.

(unless update-the-strs-coroutine
  (set! update-the-strs-coroutine
        (coroutine

Unless somehow something else is mutating it in between?

Here's the internal error:

coroutine-value: contract violation
  expected: coroutine?
  given: #f
  in: the 1st argument of
      (-> coroutine? any/c)
  contract from: 
      <pkgs>/gui-lib/framework/private/coroutine.rkt
  blaming: <pkgs>/drracket/drracket/private/syncheck/blueboxes-gui.rkt
   (assuming the contract is correct)
  at: <pkgs>/gui-lib/framework/private/coroutine.rkt:10.3
  context...:
   .../collects/racket/contract/private/blame.rkt:163:0: raise-blame-error16
   .../collects/racket/contract/private/arrow-val-first.rkt:388:18
   .../pkgs/drracket/drracket/private/syncheck/blueboxes-gui.rkt:446:4: update-the-strs method in ...ck/blueboxes-gui.rkt:176:2
   .../pkgs/drracket/drracket/private/syncheck/blueboxes-gui.rkt:652:4: syncheck:update-blue-boxes method in ...ck/blueboxes-gui.rkt:627:2
   .../pkgs/drracket/drracket/private/syncheck/gui.rkt:2090:10: loop
   .../pkgs/gui-lib/framework/private/logging-timer.rkt:41:0: log-timeline/proc
   .../pkgs/gui-lib/mred/private/wx/common/queue.rkt:428:6
   .../pkgs/gui-lib/mred/private/wx/common/queue.rkt:479:32
   .../pkgs/gui-lib/mred/private/wx/common/queue.rkt:627:3

Start in drracket/private/syncheck/blueboxes-gui.rkt, especially lines 447-459 and the update-the-strs method, then trace the stack from coroutine-value through syncheck:update-blue-boxes. Determine how update-the-strs-coroutine can be #f at the failing call and confirm that the internal error no longer occurs when the update path runs.