[web] Fix error when opening child thread from robotext

Summary:
We don't have thread in messageStore when user is not a member
Steps to reproduce error:

1. Create thread with two users
2. Create sub thread with only one of parent thread members
3. As non-member user, open parent thread on web and click robotext message about child thread creation

Test Plan: Verify that opening thread is not causing errors

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: zrebcu411, Adrian, atul, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D679