cowboy commit -- fixing fatal I introduced from D4174

Summary: we don't always have a diff so instead set an explicit title in the controller.

Test Plan: no more fatals. grepped carefully for every call site and tested them all

src/applications/differential/controller/DifferentialDiffViewController.php

@@ -132,6 +132,8 @@ final class DifferentialDiffViewController extends DifferentialController {
       ->setVisibleChangesets($changesets)
       ->setRenderingReferences($refs)
       ->setStandaloneURI('/differential/changeset/')
+      ->setDiff($diff)
+      ->setTitle('D'.$diff->getID());
       ->setUser($request->getUser());
 
     return $this->buildStandardPageResponse(