Why is my post API call resulting in a null shop query?

[Amit24]

https://github.com/Shopify/shopify-app-template-php. I have created shopify app using this template. I defined all routes in web.php file. and I try to call post api in my dashboard page. I have established bridge connection also. Issue is that When I call post API. I got shop query null. 

Shopify\Utils::sanitizeShopDomain(): Argument #1 ($shop) must be of type string, null given, called in /app/app/Lib/AuthRedirection.php on line 19 {"exception":"[object] (TypeError(code: 0): Shopify\\Utils::sanitizeShopDomain(): Argument #1 ($shop) must be of type string, null given, called in /app/app/Lib/AuthRedirection.php on line 19 at /app/vendor/shopify/shopify-api/src/Utils.php:32)