private function addSortFields(Fieldset $fieldset)
    {
        $sortFieldsOptions = [
            ['value' => '_first', 'label' => __("First")],
            ['value' => '_last',  'label' => __("Last")],
        ];

        $fieldset->addField(
            'sort_order_asc_missing',
            'select',
            [
                'name'   => 'sort_order_asc_missing',
                'label'  => __('Sort products without value when sorting ASC'),
                'values' => $sortFieldsOptions,
                'note'   => __('How the products which are missing values for this attribute should be treated when using it to sort.'),
            ],
            'used_for_sortby'
        );

        $fieldset->addField(
            'sort_order_desc_missing',
            'select',
            [
                'name'   => 'sort_order_desc_missing',
                'label'  => __('Sort products without value when sorting DESC'),
                'values' => $sortFieldsOptions,
                'note'   => __('How the products which are missing values for this attribute should be treated when using it to sort.'),
            ],
            'sort_order_asc_missing'
        );

        return $this;
    }