Error Call to a member function on null

I’ve created custom module for event and observer to restrict some city from COD.
However I’m facing an error at checkout page and inside the log I found the following error:

Error(code: 0): Call to a member function getShippingAddress() on null

Observer :

<?php
namespace VendorModuleObserver;
use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkAppRequestDataPersistorInterface;
use MagentoFrameworkAppObjectManager;

class PaymentMethodAvailable implements ObserverInterface
{
    protected $_appState;

    public function __construct(
        MagentoFrameworkAppState $appState
    ) {
        $this->_appState = $appState;
    }

    public function execute(MagentoFrameworkEventObserver $observer)
    {
        $result = $observer->getEvent()->getResult();
        $method_instance = $observer->getEvent()->getMethodInstance();
        $Shipping =$observer->getEvent()->getQuote()->getShippingAddress();

        $quote = $observer->getEvent()->getQuote();
          /* 
          *  During Checkout magento call payment methods from both
          *   area frontend and Web_api are
          */

            if($method_instance->getCode() == 'cashondelivery')
            {

            if( strtolower($Shipping['city']) == 'liverpool'){
                    $result->setData('is_available',true); 
                }
                else{

                    $result->setData('is_available',false); 
                }
            }
        
    }      
}