Decode a String futter

Why String can’t decode back to Map after change to String?

FirebaseMessaging.onMessage.listen((RemoteMessage message) {
      var convertToString = (; 
      var map = jsonDecode(convertToString);


E/flutter (17220): [ERROR:flutter/lib/ui/] Unhandled Exception: FormatException: Unexpected character (at character 2)
E/flutter (17220): {notification_type: abc, ABC: {"assigned_to_id":null,"descri...

Here my message data

{notification_type: abc, ABC: {...}, sound: default, body: ..., title: ...}

I fix it using jsonEncode instead of toString.