How to debug broken cache in Enterprise 1.14.2.0 with FPC enabled?

Question

I have a strange problem on one Magento Enterprise project (v. 1.14.2.0). Few hours after cache cleaning customers saw white page while customer trying to login for guest user it's working

Fatal error: Uncaught Error: Call to a member function setSkipRenderTag() on boolean in /var/www/project/app/code/core/Enterprise/PageCache/Model/Container/Minicart.php on line 41 Call Stack: 0.0002 645808 
1. {main}() /var/www/project/index.php:0 0.0006 786880 
2. Mage::run() /var/www/project/index.php:91 0.0015 1151072 
3. Mage_Core_Model_App->run() /var/www/project/app/Mage.php:684 0.0811 5079848 
4. Mage_Core_Controller_Varien_Front->dispatch() /var/www/project/app/code/core/Mage/Core/Model/App.php:354 0.0845 5538080 
5. Mage_Core_Controller_Varien_Router_Standard->match() /var/www/project/app/code/core/Mage/Core/Controller/Varien/Front.php:172 0.0855 5590776 
6. Mage_Core_Controller_Varien_Action->dispatch() /var/www/project/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php:254 0.1336 8047848 
7. Enterprise_PageCache_RequestController->processAction() /var/www/project/app/code/core/Mage/Core/Controller/Varien/Action.php:418 0.1604 8722832 
8. Enterprise_PageCache_Model_Container_Abstract->applyInApp() /var/www/project/app/code/core/Enterprise/PageCache/controllers/RequestController.php:38 0.1604 8722832 
9. Enterprise_PageCache_Model_Container_Minicart->_renderBlock() /var/www/project/app/code/core/Enterprise/PageCache/Model/Container/Abstract.php:109

Method form Enterprise_PageCache_Model_Container_Minicart class.

protected function _renderBlock()
{
    $layout = $this->_getLayout('default');
    $block = $layout->getBlock('minicart_head');
    $block->setSkipRenderTag(true);        // this line throws exception
    return $block->toHtml();
}

Answer 1

This issue can be solved by magento EE SUPEE 6186 patch.

Magento provide SUPEE 6186 patch to solve this issue