racket/gui cocoa: fix crashing bug with combo-field%

Merge to v6.0
(cherry picked from commit 9a120f6acc)

pkgs/gui-pkgs/gui-lib/mred/private/wx/cocoa/canvas.rkt

@@ -692,12 +692,13 @@
        #t)
      (define/public (on-combo-select i) (void))
      (define/public (popup-combo)
-       ;; Pending refresh events intefere with combo popups
+       ;; Pending refresh events interfere with combo popups
        ;; for some reason, so flush them:
        (yield-refresh)
        (flush)
        ;; Beware that the `popUp:' method is undocumented:
-       (tellv (tell content-cocoa cell) popUp: #f))
+       (atomically
+        (tellv (tell content-cocoa cell) popUp: #f)))
 
      (define clear-bg? (and (not (memq 'transparent canvas-style)) 
                             (not (memq 'no-autoclear canvas-style))))

pkgs/gui-pkgs/gui-test/tests/gracket/item.rkt

@@ -1708,7 +1708,9 @@
 (define (combo-frame empty?)
   (define f (make-frame frame% "Combo Test"))
   (define p f)
-  (define actual-content '("Apple" "Banana"))
+  (define actual-content (if empty?
+                             null
+                             '("Apple" "Banana")))
   (define (callback c e) (void))
   (define c (make-object (class combo-field% 
                            (define/override (on-popup e)
@@ -2402,6 +2404,7 @@
 (make-object button% "Make Choice Frame" cp (lambda (b e) (choice-or-list-frame #f null #f)))
 (make-object button% "Make Empty Choice Frame" cp (lambda (b e) (choice-or-list-frame #f null #t)))
 (make-object button% "Make Combo Frame" cp (lambda (b e) (combo-frame #f)))
+(make-object button% "Make Empty Combo Frame" cp (lambda (b e) (combo-frame #t)))
 (define lcp (make-object horizontal-pane% ap))
 (send lcp stretchable-width #f)
 (define list-columns-choice (new choice%