Prevents errors when there is no successor field. Refs #8859, #8679.
Prevents errors when there is no successor field. Refs #8859, #8679.

git-svn-id: 2159781fc7c14c6bdd1f7e265daf7a25099c31e7@10389 7322e99d-02ea-0310-aa39-e9a107903beb

trac-hacks/trac-jsGantt,trac-hacks/trac-jsGantt,trac-hacks/trac-jsGantt