fix persistence bugs and update example

packages/stream_chat_persistence/example/lib/main.dart

@@ -7,6 +7,8 @@ Future<void> main() async {
   /// project dashboard.
   final client = StreamChatClient('b67pax5b2wdq');
 
+  WidgetsFlutterBinding.ensureInitialized();
+
   /// Set the chatPersistenceClient for offline support
   client.chatPersistenceClient = StreamChatPersistenceClient(
     logLevel: Level.INFO,

packages/stream_chat_persistence/lib/src/dao/message_dao.dart

@@ -3,6 +3,7 @@ import 'package:stream_chat/stream_chat.dart';
 import 'package:stream_chat_persistence/src/db/moor_chat_database.dart';
 import 'package:stream_chat_persistence/src/entity/messages.dart';
 import 'package:stream_chat_persistence/src/entity/users.dart';
+
 import '../mapper/mapper.dart';
 
 part 'message_dao.g.dart';
@@ -136,7 +137,11 @@ class MessageDao extends DatabaseAccessor<MoorChatDatabase>
 
   /// Updates the message data of a particular channel with
   /// the new [messageList] data
-  Future<void> updateMessages(String cid, List<Message> messageList) {
+  Future<void> updateMessages(String cid, List<Message> messageList) async {
+    if (messageList == null) {
+      return;
+    }
+
     return batch((batch) {
       batch.insertAll(
         messages,

packages/stream_chat_persistence/lib/src/mapper/message_mapper.dart

@@ -47,7 +47,7 @@ extension MessageX on Message {
   MessageEntity toEntity({String cid}) {
     return MessageEntity(
       id: id,
-      attachments: attachments.map((it) => jsonEncode(it)).toList(),
+      attachments: attachments?.map((it) => jsonEncode(it))?.toList() ?? [],
       channelCid: cid,
       type: type,
       parentId: parentId,