Magento Upgrade – Fatal error: Call to a member function getUsername() on a non-object in

Today I upgraded a client to the latest version of Magento (1.7.0.2).  Upon finishing the FTP of the new version, I could see the front end of the website with no problem.  The backend, however, would not work becuase of this error: Fatal error: Call to a member function getUsername() on a non-object in /app/design/adminhtml/default/default/template/page/header.phtml…

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