Crash on Backup Notes

Tomdroid (beta) crashes when trying backup notes when there aren't notes.

The obtained exception stacktrace is reported below:

E/AndroidRuntime( 535): FATAL EXCEPTION: main
E/AndroidRuntime( 535): java.lang.NullPointerException
E/AndroidRuntime( 535): at org.tomdroid.sync.sd.SdCardSyncService.backupNotes(SdCardSyncService.java:443)
E/AndroidRuntime( 535): at org.tomdroid.ui.PreferencesActivity$8$1.onClick(PreferencesActivity.java:277)
E/AndroidRuntime( 535): at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:159)
E/AndroidRuntime( 535): at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 535): at android.os.Looper.loop(Looper.java:123)
E/AndroidRuntime( 535): at android.app.ActivityThread.main(ActivityThread.java:3683)
E/AndroidRuntime( 535): at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 535): at java.lang.reflect.Method.invoke(Method.java:507)
E/AndroidRuntime( 535): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
E/AndroidRuntime( 535): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
E/AndroidRuntime( 535): at dalvik.system.NativeStart.main(Native Method)
W/ActivityManager( 68): Force finishing activity org.tomdroid/.ui.PreferencesActivity