fixed tests, null errors

packages/stream_chat/lib/src/models/attachment_file.dart

@@ -87,7 +87,7 @@ class AttachmentFile {
   final int? size;
 
   /// File extension for this file.
-  String? get extension => name?.split('.')?.last;
+  String? get extension => name?.split('.').last;
 
   /// Serialize to json
   Map<String, dynamic> toJson() => _$AttachmentFileToJson(this);

packages/stream_chat/lib/src/models/channel_state.dart

@@ -43,8 +43,8 @@ class ChannelState {
   final List<Read>? read;
 
   /// Create a new instance from a json
-  static ChannelState fromJson(Map<String, dynamic>? json) =>
-      _$ChannelStateFromJson(json!);
+  static ChannelState fromJson(Map<String, dynamic> json) =>
+      _$ChannelStateFromJson(json);
 
   /// Serialize to json
   Map<String, dynamic> toJson() => _$ChannelStateToJson(this);

packages/stream_chat/lib/src/models/serialization.dart

@@ -11,7 +11,7 @@ class Serialization {
 
   /// List of users to list of userIds
   static List<String?>? userIds(List<User>? users) =>
-      users?.map((u) => u.id)?.toList();
+      users?.map((u) => u.id).toList();
 
   /// Takes unknown json keys and puts them in the `extra_data` key
   static Map<String, dynamic>? moveToExtraDataFromRoot(