关于AIDL跨进程调用服务端的方法问题

调用这个服务端的removeTrack方法报错说是空引用怎么解决
long Id =imediaPlaybackService.removeTrack(num1 );

 java.lang.NullPointerException: Attempt to invoke interface method 'int com.android.music.IMediaPlaybackService.removeTrack(long)' on a null object reference

http://lydia-fly.iteye.com/blog/2024575

他是说你这个imediaPlaybackService是空