yii\behaviors\SluggableBehavior::init PHP Method

    public function init()
    {
        parent::init();
        if (empty($this->attributes)) {
            $this->attributes = [BaseActiveRecord::EVENT_BEFORE_VALIDATE => $this->slugAttribute];
        }
        if ($this->attribute === null && $this->value === null) {
            throw new InvalidConfigException('Either "attribute" or "value" property must be specified.');
        }
    }