Warning: Illegal offset type in isset or empty in vendor/magento/module-email/Model/Template/Config.php on line 224

I am creating custom module to process order template. In Magento 1,It is working fine with below code,

$template = 'sales_email_order_template';
$emailTemplate  = Mage::getModel('core/email_template')
                      ->loadDefault($template);

When i use loaddefault method in the Magento 2, it shows me an error
like, Warning: Illegal offset type in isset or empty in
/var/www/html/xyz/vendor/magento/module-email/Model/Template/Config.php
on line 224 []

Please check my code below,

class Orderdetail extends MagentoFrameworkViewElementTemplate{

    protected $order;
    protected $templateFactory;
    protected $_storeManager;
    protected $_paymentHelper;

    public function __construct(
        MagentoBackendBlockTemplateContext $context,
        MagentoSalesApiDataOrderInterface $order,
        MagentoEmailModelTemplateFactory $templateFactory,
        MagentoStoreModelStoreManagerInterface $storeManager,
        MagentoPaymentHelperData $paymentHelper,
        array $data = []
    )
    {
        $this->order = $order;
        $this->templateFactory = $templateFactory;
        $this->_storeManager = $storeManager;
        $this->_paymentHelper = $paymentHelper;
        parent::__construct($context, $data);
    }

  public function getOrderDataHtml(){
    echo $orderid = $this->getRequest()->getParam('orderid');
     $order = $this->order->load($orderid);
     if ($order->getCustomerIsGuest()) {
            $template = 'sales_email_order_guest_template';
        } else {
            $template = 'sales_email_order_template';
        }
        $template = $this->templateFactory->create();
        $emailTemplate  = $template->loadDefault($template);
        
        $emailTemplateVars = array();       
        $emailTemplateVars['store'] = $this->_storeManager->getStore();        

        // order you want to load by ID
        $emailTemplateVars['order'] = $order;

        // Template Variables
        $order = $emailTemplateVars['order'];
        $paymentBlock = $this->_paymentHelper->getInfoBlock($order->getPayment())
                        ->setIsSecureMode(true);
        $paymentBlock->getMethod()->setStore($this->_storeManager->getStore()); 

        $emailTemplateVars['payment_html'] = $paymentBlock->toHtml();

        //displays the rendered email template
        return $emailTemplate->getProcessedTemplate($emailTemplateVars);
    
  }

Please help me. Your help would be appreciated!!!

Thanks