Fix an issue where the Maniphest burnup chart was trying to render a non-View object

Summary:
See PHI1714. This code is incorrectly rendering the chart panel twice, sort of, and passing a non-View object to rendering.

After D21044, this fatals by raising an exception in rendering.

Test Plan: Loaded page, no more exception.

Differential Revision: https://secure.phabricator.com/D21185