The following _TypeError was thrown while handling a gesture: type 'Null' is not a subtype of type 'String' flutter

Question

I get this error whenever I press the send message button:

My code below:

  void sendMessage(String text) {
    var messageJson = {
      "message": text,
      "isMe": socket.id,
    };
    socket.emit('message', messageJson);
    chatController.chatMessages.add(Message.fromJson(messageJson));
  }

  void setUpSocketListener() {
    socket.on('message-receive', (data) {
      print(data);
      chatController.chatMessages.add(Message.fromJson(data));
    });
  }
}

This is the model I made:

class Message {
  String message;
  String isMe;

  Message({required this.message, required this.isMe});

  factory Message.fromJson(Map<String, dynamic> json) {
    return Message(message: json["message"], isMe: json["isMe"]);
  }
}

I don't know what the problem is.

Additional code: