Silence an undeclared variable warning

Summary:
If you don't have a custom renderer, this variable does not get set
and emits a warning when you try to read it.

Test Plan:
Loaded page before and after change, warnings went away (lines
145 and 154 in old file).

Reviewed By: aran
Reviewers: tuomaspelkonen, aran
CC: jungejason, aran, epriestley
Differential Revision: 186