Fixed Bugs:
E/AndroidRuntime(19159): java.lang.RuntimeException: Error receiving broadcast Intent { act=android.intent.action.SCREEN_OFF flg=0x50000010 } in com.example.tracker.ScreenReceiver@41e9f6e0
 E/AndroidRuntime(19159): at com.example.tracker.AggregateMessages.getMessages(AggregateMessages.java:57)
Solution: Handle the null pointer.