R1:b71e08966934

Fix a fatal when viewing methods which no longer exist in the Conduit call log

Summary: Fixes T12252.

Test Plan:
I just faked this, but likely repro is:

- Call method `x.y`.
- Remove method `x.y` from the codebase.
- View log.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12252

Differential Revision: https://secure.phabricator.com/D17342