Fix crash when add after empty

[takahirom]

App crashes when you select 'Add' after the 'Empty'. I was fixed it. And, I have modified to be able to add.

Crash stacktrace

E/AndroidRuntime(22443): java.lang.IllegalArgumentException: maxValue must be >= 0
E/AndroidRuntime(22443):        at android.widget.NumberPicker.setMaxValue(NumberPicker.java:1437)
E/AndroidRuntime(22443):        at com.example.android.recyclerplayground.NumberPickerDialog.setPickerRange(NumberPickerDialog.java:48)
E/AndroidRuntime(22443):        at com.example.android.recyclerplayground.fragments.RecyclerFragment.onOptionsItemSelected(RecyclerFragment.java:70)
E/AndroidRuntime(22443):        at android.support.v4.app.Fragment.performOptionsItemSelected(Fragment.java:1894)

[devunwired]

Thanks.