Symptoms
The following error message is shown or logged on attempt to edit products, categories or manage attributes in Magento Backend (blank page is shown if displaying error messages is disabled) after Magento upgrade or extension installation:
PHP Fatal error: Call to a member function toOptionArray() on a non-object in app/code/core/Mage/Adminhtml/Block/System/Config/Form.php on line 385
Cause
The most probable reason of this issue is not-upgraded Mage_Core_Adminhtml package or package is upgraded but old code is cached by compiler.
Solution
Make sure to disable compiler before any upgrades or extension installation. To disable compiler and rebuild compiled files please refer to How do I disable Magento Compiler article.
If the issue persist after disabling compiler please ensure that Mage_Core_Adminhtml package have the same version as Mage_Core package in Magento Connect Manager. It can be checked/upgraded at System > Magento Connect > Magento Connect Manager.
