Magento 2 : Error when getting sub categories

Question

I have this function in my block.

public function getSubcategoryByUrlkey($url_key)
{   

    $categoryCollection = $this->_categoryCollectionFactory->create()      
                ->addAttributeToFilter('url_key',$url_key)->getFirstItem(); 

    echo $categoryCollection->getSelect(); exit;

    return $childs = $categoryCollection->getChildrenCategories();

}

I cannot view anything from this collection. My Magento version is Magento ver. 2.2.2. What is the mistake I am doing here?

echo $categoryCollection->getSelect();

this also not retrieve the query

Update I am getting this error

Fatal error: Uncaught Error: Call to undefined method Magento\Catalog\Model\ResourceModel\Category\Collection::getChildrenCategories()

Answer 1

Remove ->getFirstItem() then it will show the query

$categoryCollection = $this->_categoryCollectionFactory->create()      
            ->addAttributeToFilter('url_key',$url_key); 

echo $categoryCollection->getSelect(); exit;