protected void enableStartButtonIfReady() {
        binding.start.setEnabled(StringUtils.isNotEmpty(binding.username.getText()) &&
                StringUtils.isNotEmpty(binding.password.getText()));
    }
Enable or disable the start button depending on login/password field.
If both fields are not empty, button is enabled.
main/src/main/java/cgeo/geocaching/activity/TokenAuthorizationActivity.java