Correct a method signature in DifferentialDraftField
Summary:
Ref T12190. See https://discourse.phabricator-community.org/t/exception-preventing-access-to-differential-application/606.
(I have a followup to fix the root issue.)
Test Plan: Loaded Differential with an eye on the error log in PHP7, no longer saw warnings.
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T12190
Differential Revision: https://secure.phabricator.com/D18723