Fix a fatal in "create subtask"

Summary: This got clipped by the actor/email thing I think.

Test Plan: "Create Subtask" no longer fatals.

Reviewers: btrahan, vrana, hsb

Reviewed By: hsb

CC: aran, lesliepc16

Maniphest Tasks: T1918

Differential Revision: https://secure.phabricator.com/D3718