As we seen with my previous blog upgrade community edition magento 1.7 to magento 1.8, we know how to upgrade magento with latest version.
When I was doing the same with one my project, I come across the error something like,
Fatal error: Call to a member function addCacheTag() on a non-object in (Base Path)/app/code/core/Mage/Catalog/Model/Observer.php on line 215
For quick fix, initially I have comment out two lines from file app/code/core/Mage/Catalog/Model/Observer.php
Line # 241 ($menuBlock->addModelTags($categoryModel);) Line # 215 ($block->addCacheTag(Mage_Catalog_Model_Category::CACHE_TAG);)
But after spending some time, I come to know that I have used one of the menu extension which is exptending the “Mage_Page_Block_Html_Topmenu” and there is some problem with Mage::dispatchEvent.
With my extended code it was just something like
Mage::dispatchEvent('page_block_html_topmenu_gethtml_before', array( 'menu' => $this->_menu ));
Updated code is,
Mage::dispatchEvent('page_block_html_topmenu_gethtml_before', array( 'menu' => $this->_menu, 'block' => $this //add this line ));
Most probebly this would resolve your issue.