the autosave file might be deleted by the user after it is created

the first time but before the file is actually saved. guard against that.

closes PR 12698

collects/framework/private/editor.rkt

@@ -598,7 +598,8 @@
                (when (is-a? this text%)
                  (send this set-file-format orig-format))
                (when old-auto-name
-                 (delete-file old-auto-name))
+                 (when (file-exists? old-auto-name)
+                   (delete-file old-auto-name)))
                (set! auto-saved-name auto-name)
                (set! auto-save-out-of-date? #f)
                auto-name))]