Picker loads with month-picker-previous and month-picker-next buttons disabled.

The problem seems to be that _value is null (not false, but falsie) on input.
The check if (_btnWidget.option('disabled') !== _value) is true, so _btnWidget.option('disabled', _value) executes, which disables the buttons.

An easy fix is to validate _value:
_value = !(!_value);