Clean up an issue with meta-editing of edit engines
Ref T9908. These meta-edit-engines are used to generate the main editengine UIs, but they're also editable.
Fix an exception when trying to edit the meta editengine.
Test Plan: Edited editengineconfiguration editengine.
Reviewed By: chad
Maniphest Tasks: T9908
Differential Revision: https://secure.phabricator.com/D14783