Avoid RejectedExecutionException in lein :(

(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: lawrence@krubner.com

I am curious what Dave Ray wrote here in his Seesaw repo.

(defn -main [& args]
  (when-not (first args)
    (println "Usage: gaidica ")
    (System/exit 1))
  (reset! api-key (first args))
  (invoke-later
    (->
      (make-frame)
      add-behaviors
      show!))
  ; Avoid RejectedExecutionException in lein :(
  @(promise))

Does anyone know what this about? What causes RejectedExecutionException in lein?

Source