org-timer.el (org-timer-stop): Set `org-timer-current-timer' to nil

* org-timer.el (org-timer-stop): Set `org-timer-current-timer'
to nil.

lisp/org-timer.el

@@ -161,7 +161,8 @@ With prefix arg STOP, stop it entirely."
   (interactive)
   (run-hooks 'org-timer-stop-hook)
   (setq org-timer-start-time nil
-	org-timer-pause-time nil)
+	org-timer-pause-time nil
+	org-timer-current-timer nil)
   (org-timer-set-mode-line 'off)
   (message "Timer stopped"))