Summary: This prevents crashes when looking at builds, where the build steps have been deleted on the build plan since the build was run. Currently the only information that's pulled from the build step is the description (because this was too large to copy to every target).
Test Plan: Tested it locally.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D10361