[Messaging] What is FirebaseMessaging.instance.sendMessage for ?

[ash-hashtag]

throws this when used

type 'Null' is not a subtype of type 'String' in type cast
I/flutter (17257): #0      new LinkedHashMap.from.<anonymous closure> (dart:collection/linked_hash_map.dart:187:26)
I/flutter (17257): #1      _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:617:13)
I/flutter (17257): #2      new LinkedHashMap.from (dart:collection/linked_hash_map.dart:186:11)
I/flutter (17257): #3      platformExceptionToFirebaseException
package:_flutterfire_internals/src/exception.dart:42
I/flutter (17257): #4      convertPlatformExceptionToFirebaseException
package:_flutterfire_internals/src/exception.dart:27
I/flutter (17257): #5      convertPlatformException
package:firebase_messaging_platform_interface/…/utils/exception.dart:15
I/flutter (17257): #6      MethodChannelFirebaseMessaging.sendMessage

I can't find in docs anywhere what this is supposed to be? it just has one line
"Send a new [RemoteMessage] to the FCM server"

and I don't understand it. So can we do something when it is sent to FCM server, Idk any FCM server side programming, can we listen to these messages in server? I thought maybe it can send message to a fcm token, or a topic through the server, but it wasn't even working for me to start with.

Thanks