Backtrace

$ java -cp clojure.jar clojure.main ~/Projects/Clojure/fake-cs.clj 
Sending...
#<Agent@1f758500: {:name Music, :online false}>

Starting...
Exception in thread "main" java.lang.NullPointerException (fake-cs.clj:0)
	at clojure.lang.Compiler.eval(Compiler.java:5440)
	at clojure.lang.Compiler.load(Compiler.java:5857)
	at clojure.lang.Compiler.loadFile(Compiler.java:5820)
	at clojure.main$load_script.invoke(main.clj:221)
	at clojure.main$script_opt.invoke(main.clj:273)
	at clojure.main$main.doInvoke(main.clj:354)
	at clojure.lang.RestFn.invoke(RestFn.java:408)
	at clojure.lang.Var.invoke(Var.java:365)
	at clojure.lang.AFn.applyToHelper(AFn.java:161)
	at clojure.lang.Var.applyTo(Var.java:482)
	at clojure.main.main(main.java:37)
Caused by: java.lang.NullPointerException
	at user$eval2$send_start__3.invoke(fake-cs.clj:16)
	at user$eval2.invoke(fake-cs.clj:17)
	at clojure.lang.Compiler.eval(Compiler.java:5424)
	... 10 more

fake-cs.clj

(def my-service (agent {:name "Music" :online false}))

(defn bring-online [service]
  (cond
   (service :online) (do
                       (println "Already started...")
                       service)
   :else  (do
            (println "Starting...")
            (.. Thread (sleep 1000))
            (assoc service :online (not (service :online))))))

(let [send-start #((println "Sending...")
                   (println my-service)
                   (println)
                   (send my-service bring-online))]
  (send-start)
  (send-start))

(shutdown-agents)

(println @my-service)