[native] Fixed crash when logging out while thread opened

Summary: Removed invariant, which was causing the error, and checked null values in measureMessages method.

Test Plan: Checked logging out while there was opened the chat window, not it doesn't throw an error.

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6665