[lib] fix creating thin threads with multimedia

Summary: ENG-9346.

Test Plan:

1. Creating thin thread.
2. Creating thick thread.
3. Creating thin sidebar.
4. Creating thick sidebar.

Tested both web and native.

Reviewers: tomek, bartek

Reviewed By: tomek, bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13406