Fix error when creating sidebar from past user message

Summary: Details: https://www.notion.so/commapp/The-native-app-shows-an-error-when-attempting-to-create-a-sidebar-on-a-removed-past-user-36dec6733aa34e49a10180fb7ddc3d6d

Test Plan: Check if creating threads and sidebars works as before, and if bug is fixed

Reviewers: ashoat, palys-swm

Reviewed By: ashoat, palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D712