Upgrading Laravel 5.5 to 5.6 gives error on PackageManifest.php

[krixnaas]

Laravel Version: 5.6.*
PHP Version: 7.4
Description:
Upgrading steps by steps until Laravel version-8. Gives the error in PackageManifest.php.
I wonder how GitHub file of Laravel https://github.com/laravel/framework/blob/5.6/src/Illuminate/Foundation/PackageManifest.php
is different than composer installed file "laravel/framework": "5.6.*" on local machine.

Steps To Reproduce:
Tried solution for updating as found on Stackoverflow:
changing files as shown, 65a5551

leads to
\vendor\laravel\framework\src\Illuminate\Container\Container.php:826
822|
823| // If the class is null, it means the dependency is a string or some other
824| // primitive type which we can not resolve since it is not a class and
825| // we will just bomb out with an error since we have no-where to go.

826| $results[] = is_null($dependency->getClass())
827| ? $this->resolvePrimitive($dependency)
828| : $this->resolveClass($dependency);
829| }

1 ReflectionParameter::getClass()
C:\xampp\htdocs\root\betengine\vendor\laravel\framework\src\Illuminate\Container\Container.php:826

2 Illuminate\Container\Container::resolveDependencies()
C:\xampp\htdocs\root\betengine\vendor\laravel\framework\src\Illuminate\Container\Container.php:795

Also Tried:

Composer dump-autoload
downgrading composer versions

It won't work.
Any help is appreciated.