I found that this error is due to wrong call for $html->url() method. I was call it as $this->url('/img/logo.png', "full"), but it should be $html->url('/img/logo.png', true).
The call came from an element file called head.ctp. the following is the full contest warning explaonation:
array_merge - [internal], line ??
Router::url() - ROOT\cakephp\cake\libs\router.php, line 754
Helper::url() - ROOT\cakephp\cake\libs\view\helper.php, line 177
include - APP\views\themed\slate\elements\head.ctp, line 15 //HERE IS THE Mistaken Call
View::_render() - ROOT\cakephp\cake\libs\view\view.php, line 665
View::element() - ROOT\cakephp\cake\libs\view\view.php, line 342
include - APP\views\themed\slate\layouts\blog.ctp, line 1
View::_render() - ROOT\cakephp\cake\libs\view\view.php, line 665
View::renderLayout() - ROOT\cakephp\cake\libs\view\view.php, line 453
View::render() - ROOT\cakephp\cake\libs\view\view.php, line 387
Controller::render() - ROOT\cakephp\cake\libs\controller\controller.php, line 808
Dispatcher::_invoke() - ROOT\cakephp\cake\dispatcher.php, line 229
Dispatcher::dispatch() - ROOT\cakephp\cake\dispatcher.php, line 193
[main] - APP\webroot\index.php, line 88
Warning (2): extract() expects parameter 1 to be array, null given [ROOT\cakephp\cake\libs\router.php, line 754]
Notice (8): Undefined variable: escape [ROOT\cakephp\cake\libs\router.php, line 930