Clean up an issue with meta-editing of edit engines

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9908

Differential Revision: https://secure.phabricator.com/D14783