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 463

Go to app\code\core\Mage\Adminhtml\Block\System\Config\Form.php

find the following on line 463

$optionArray = $sourceModel->toOptionArray($fieldType == 'multiselect');
and replace it with the following:

if(is_object($sourceModel)){
$optionArray = $sourceModel->toOptionArray($fieldType == 'multiselect');
}