Class from observer "quickview_check_referer_url" is not initialized

#0 /home/lanmaster/matreshka/www/app/code/core/Mage/Core/Model/App.php(1498): Mage::throwException()
#1 /home/lanmaster/matreshka/www/app/code/core/Mage/Core/Model/App.php(1466): Mage_Core_Model_App->_callObserverMethod()
#2 /home/lanmaster/matreshka/www/app/Mage.php(526): Mage_Core_Model_App->dispatchEvent()
#3 /home/lanmaster/matreshka/www/app/code/core/Mage/Core/Controller/Varien/Action.php(299): Mage::dispatchEvent()
#4 /home/lanmaster/matreshka/www/app/code/core/Mage/Catalog/controllers/CategoryController.php(135): Mage_Core_Controller_Varien_Action->loadLayoutUpdates()
#5 /home/lanmaster/matreshka/www/app/code/local/Amasty/Shopby/controllers/CategoryController.php(49): Mage_Catalog_CategoryController->viewAction()
#6 /home/lanmaster/matreshka/www/app/code/core/Mage/Core/Controller/Varien/Action.php(424): Amasty_Shopby_CategoryController->viewAction()
#7 /home/lanmaster/matreshka/www/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(261): Mage_Core_Controller_Varien_Action->dispatch()
#8 /home/lanmaster/matreshka/www/app/code/core/Mage/Core/Controller/Varien/Front.php(179): Mage_Core_Controller_Varien_Router_Standard->match()
#9 /home/lanmaster/matreshka/www/app/code/core/Mage/Core/Model/App.php(351): Mage_Core_Controller_Varien_Front->dispatch()
#10 /home/lanmaster/matreshka/www/app/Mage.php(766): Mage_Core_Model_App->run()
#11 /home/lanmaster/matreshka/www/index.php(55): Mage::run()
#12 {main}