fix: add null guards to messages page

- Guard against null response from chatApi.listConversations()
- Use fallback empty array for conversations state
- Prevents 'Cannot read properties of null' errors

frontend/src/app/dashboard/messages/page.tsx

@@ -50,9 +50,11 @@ export default function AdminMessagesPage() {
 
     try {
       const data = await chatApi.listConversations()
-      setConversations(data)
-      if (data.length > 0 && !selectedConversation) {
-        setSelectedConversation(data[0])
+      // Guard against null/undefined response
+      const safeData = data || []
+      setConversations(safeData)
+      if (safeData.length > 0 && !selectedConversation) {
+        setSelectedConversation(safeData[0])
       }
       setLoading(false)
       setError(null)
@@ -129,7 +131,7 @@ export default function AdminMessagesPage() {
     }
   }, [selectedConversation, serviceConfigured])
 
-  const filteredConversations = conversations.filter((conv) =>
+  const filteredConversations = (conversations || []).filter((conv) =>
     (conv.participantName || "Unknown").toLowerCase().includes(searchTerm.toLowerCase()),
   )