[lib] fix reading messageIDs of undefined

Summary: Fix for D13162.

Test Plan: Being able to create a thread/sidebar.

Reviewers: tomek, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13183

lib/shared/message-utils.js

@@ -726,7 +726,7 @@ function useFetchMessages(threadInfo: ThreadInfo): () => Promise<void> {
 
   const threadID = threadInfo.id;
   const messageIDs = useSelector(
-    state => state.messageStore.threads[threadID].messageIDs,
+    state => state.messageStore.threads[threadID]?.messageIDs,
   );
 
   return React.useCallback(async () => {