Summary: Trying to access a non-existing edit engine "foo" via the URI `/transactions/editengine/foo/` throws an unhandled exception. `PhabricatorEditEngine::getByKey($viewer, $engine_key)` is null as `$engine_key` is non-existing `foo`. Thus avoid calling `setViewer()` on `null` by splitting into a separate step after a null check. When null, show a proper 404 error instead. ``` EXCEPTION: (Error) Call to a member function setViewer() on null at [<phorge>/src/applications/transactions/controller/PhabricatorEditEngineConfigurationListController.php:17] ``` Closes T15793 Test Plan: While logged in and with sufficient permissions, * go to non-existing http://phorge.localhost/transactions/editengine/whatever/ before and after applying patch, * go to existing http://phorge.localhost/transactions/editengine/maniphest.task/ to see that existing stuff still works. Reviewers: O1 Blessed Committers, valerio.bozzolan Reviewed By: O1 Blessed Committers, valerio.bozzolan Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T15793 Differential Revision: https://we.phorge.it/D25597