racket/sandbox: fix guard on attempt to flush sandbox plumber

pkgs/racket-pkgs/racket-test/tests/racket/sandbox.rktl

@@ -666,6 +666,26 @@
         r1)
   (test #t equal? r1 r2))
 
+;; ----------------------------------------
+;; Check that sandbox is not flushed if
+;; its custodian has been shut down:
+
+(let ()
+  (define p (make-plumber))
+
+  (define e0
+    (parameterize ([current-plumber p])
+      (call-with-trusted-sandbox-configuration
+       (lambda ()
+         (make-evaluator 'racket/base)))))
+
+  (e0 '(require racket/sandbox))
+  (e0 '(make-evaluator 'racket/base))
+
+  (kill-evaluator e0)
+  ;; e's plumber should not be flushed:
+  (plumber-flush-all p))
+
 ;; ----------------------------------------
 
 ;; Backup test for one in "thread.rktl", since this sandbox test

pkgs/sandbox-lib/racket/sandbox.rkt

@@ -1032,7 +1032,12 @@
            (let ([p (make-plumber)])
              (define fh (plumber-add-flush! (current-plumber)
                                             (lambda (fh)
-                                              (unless terminated? 
+                                              (unless (or terminated?
+                                                          ;; The evaluator thread may have terminated
+                                                          ;; asynchronously, such as through an enclosing
+                                                          ;; custodian's shutdown
+                                                          (and user-thread
+                                                               (thread-dead? user-thread)))
                                                 (call-in-sandbox-context
                                                  evaluator
                                                  (lambda ()