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

original commit: 35e818ae50eea159f10c996c3475198bc321c662

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 auto-save-out-of-date?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))]