add to cart programmatically doesn't work

Question

I'm building rest api to dd product to cart but it doesn't work and returns quote = null.

here is my code

 /**
* @api
* @param int $productId
* @return \Magento\Quote\Model\Quote
*/
 public function addItem($productId){
    $customer = null;
    $isGuest = true;
    if($this->isLoggedIn()){
        $customerId = $this->userContext->getUserId();
        $isGuest = false;
    }             

    $_product = $this->productRepository->getById($productId); 
    $customerId =  $this->userContext->getUserId();
    $quote = $this->cartObj->getQuote()->loadByCustomer($customerId);
    $quote->setCustomerIsGuest($isGuest)
    $this->item->setProduct($_product);    
    $quote->addItem($this->item);  
    return $quote;
}


 /**
* @return bool
*/
public function isLoggedIn(){
    $customerId = $this->userContext->getUserId();
    $userType = $this->userContext->getUserType();
    if($userType == UserContextInterface::USER_TYPE_CUSTOMER){
        return true;
    }
    return false;
}