PHP8 error: method_exists() now throws an exception if the first argument is not string|object

[S0FTWEX]

Preconditions (*)

1. OpenMage 20.0.8
2. PHP 8.0.3

Steps to reproduce (*)

1. In Admin Panel, go to System -> Configuration and save any configuration you like
2. Configuration is not saved, page goes white blank with this error in log:

Uncaught TypeError: method_exists():
Argument #1 ($object_or_class) must be of type object|string, bool given in /app/code/core/Mage/Core/Model/App.php:1409
Stack trace:
#0 /app/code/core/Mage/Core/Model/App.php(1409): method_exists()
#1 /app/code/core/Mage/Core/Model/App.php(1383): Mage_Core_Model_App->_callObserverMethod()
#2 /app/Mage.php(517): Mage_Core_Model_App->dispatchEvent()
#3 /app/code/core/Mage/Core/Model/Abstract.php(515): Mage::dispatchEvent()
#4 /app/code/core/Mage/Core/Model/Abstract.php(383): Mage_Core_Model_Abstract->_afterSave()
#5 /app/code/core/Mage/Core/Model/Resource/Transaction.php(150): Mage_Core_Model_Abstract->save()
#6 /app/code/core/Mage/Adminhtml/Model/Config/Data.php(215): Mage_Core_Model_Resource_Transaction->save()

Actual result (*)

I have fixed it just like this:
edit the line 1409 in file /app/code/core/Mage/Core/Model/App.php from:

if (method_exists($object, $method)) {
to:
if (is_object($object) && method_exists($object, $method)) {

Saving system configuration is now working well.

[kiatng]

Note that the error is not specific to saving the system configuration in backend, it is thrown from Mage::dispatchEvent(), in which the classdefined in the config.xml is invalid:

magento-lts/app/code/core/Mage/Core/Model/App.php

Lines 1372 to 1391 in 28ecfb5

	foreach ($events[$eventName]['observers'] as $obsName => $obs) {
	$observer->setData(array('event'=>$event));
	Varien_Profiler::start('OBSERVER: '.$obsName);
	switch ($obs['type']) {
	case 'disabled':
	break;
	case 'object':
	case 'model':
	$method = $obs['method'];
	$observer->addData($args);
	$object = Mage::getModel($obs['model']);
	$this->_callObserverMethod($object, $method, $observer);
	break;
	default:
	$method = $obs['method'];
	$observer->addData($args);
	$object = Mage::getSingleton($obs['model']);
	$this->_callObserverMethod($object, $method, $observer);
	break;
	}

When the class cannot be created in $object = Mage::getModel($obs['model']); or $object = Mage::getSingleton($obs['model']); , $object = false.