Fix nullable errors.

samples/VirtualizationDemo/Models/Chat.cs

@@ -6,7 +6,7 @@ namespace VirtualizationDemo.Models;
 
 public class ChatFile
 {
-    public ChatMessage[] Chat { get; set; }
+    public ChatMessage[]? Chat { get; set; }
 
     public static ChatFile Load(string path)
     {

samples/VirtualizationDemo/ViewModels/ChatPageViewModel.cs

@@ -1,4 +1,5 @@
-using System.Collections.ObjectModel;
+using System;
+using System.Collections.ObjectModel;
 using System.IO;
 using VirtualizationDemo.Models;
 
@@ -9,7 +10,7 @@ public class ChatPageViewModel
     public ChatPageViewModel()
     {
         var chat = ChatFile.Load(Path.Combine("Assets", "chat.json"));
-        Messages = new(chat.Chat);
+        Messages = new(chat.Chat ?? Array.Empty<ChatMessage>());
     }
 
     public ObservableCollection<ChatMessage> Messages { get; }