protected function htmlOptionsGroup()
    {
        $class = !empty($this->defaults['groupClass']) ? $this->defaults['groupClass'] : '';
        $style = !empty($this->defaults['groupStyle']) ? $this->defaults['groupStyle'] : '';

        if (!empty($this->bootstrapDefaults['groupClass'])) {
            $class .= $this->bootstrapDefaults['groupClass'];
            if ($this->dropup) {
                $class .= ' dropup';
            }
        }
        if (!empty($this->bootstrapDefaults['groupStyle'])) {
            $style .= $this->bootstrapDefaults['groupStyle'];
        }
        if (!empty($this->groupClass) && is_string($this->groupClass)) {
            $class .= ' ' . $this->groupClass;
        }
        if (!empty($this->groupStyle) && is_string($this->groupStyle)) {
            $style .= ' ' . $this->groupStyle;
        }

        return [
            'class' => $class ?: null,
            'style' => $style ?: null,
        ];
    }