Diffusion - double check these sub objects exist before toDictionary'ing them

Summary: wordy title. Ref T3214. Not exactly sure what a valid test is here as the user has a borked setup... Shouldn't fatal though.

Test Plan: arc lint

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3214

Differential Revision: https://secure.phabricator.com/D5995