Shopify webhooks registered but not triggered when uninstall app

[zahid3d29]

Hello, shopify app developer. I am developing an app while I have been facing a problem at almost last steps of developments. 

I have succesfully registered an webhooks, we can see status after installed app.  check below...

But after uninstalled app it should triggred and removing data  from database which I request on delete.php file and also should create .log file to store response data. But nothing is happen. I think there might be a problem with delete.php file. I uploaded full delete.php file for your kind information. please help me if you can. 

<?php
require_once("inc/db_connect.php");
require_once("inc/functions.php");


define('SHOPIFY_APP_SECRET', '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'); // Replace with your SECRET KEY

function verify_webhook ($data, $hmac_header){
  $calculated_hmac = base64_encode(hash_hmac('sha256', $data, SHOPIFY_APP_SECRET, true));
  return hash_equals($hmac_header, $calculated_hmac);
}

$res = '';
$hmac_header = $_SERVER['HTTP_X_SHOPIFY_HMAC_SHA256'];
$topic_header = $_SERVER['HTTP_X_SHOPIFY_TOPIC'];
$shop_header = $_SERVER['HTTP_X_SHOPIFY_SHOP_DOMAIN'];
$data = file_get_contents('php://input');
$decoded_data = json_decode($data, true);

$verified = verify_webhook($data, $hmac_header);

if( $verified == true ) {
  if( $topic_header == 'app/uninstalled' || $topic_header == 'shop/update') {
    if( $topic_header == 'app/uninstalled' ) {

      $sql = "DELETE FROM `AccessToken` WHERE store_url='" . $shop_header . "' LIMIT 1";
      $result = mysqli_query($conn, $sql);

      $response->shop_domain = $decoded_data['shop_domain'];

      $res = $decoded_data['shop_domain'] . ' is successfully deleted from the database';
    

    } else {
      $res = $data;
    }
  }
} else {
  $res = 'The request is not from Shopify';
}

error_log('Response: '. $res); //check error.log to see the result

Let me know if you need to know something more. 

I have followed this tutorial to create webhooks. 
https://weeklyhow.com/shopify-uninstall-webhook-api-tutorial/

Best Regards

Zahid