fix: bug for rfc2833. (#534)

Author: cloudwebrtc

Diff for: lib/src/rtc_session.dart

@@ -2,6 +2,7 @@
 import 'dart:async';
 
 // Package imports:
+import 'package:collection/collection.dart';
 import 'package:flutter_webrtc/flutter_webrtc.dart';
 import 'package:sdp_transform/sdp_transform.dart' as sdp_transform;
 
@@ -95,6 +96,9 @@ class RTCSession extends EventManager implements Owner {
   // The RTCPeerConnection instance (public attribute).
   RTCPeerConnection? _connection;
 
+  // RTPSender List
+  final List<RTCRtpSender> _senders = <RTCRtpSender>[];
+
   // Incoming/Outgoing request being currently processed.
   dynamic _request;
 
@@ -168,8 +172,10 @@ class RTCSession extends EventManager implements Owner {
   @override
   int get TerminatedCode => RtcSessionState.terminated.index;
 
-  RTCDTMFSender get dtmfSender =>
-      _connection!.createDtmfSender(_localMediaStream!.getAudioTracks()[0]);
+  RTCDTMFSender? get dtmfSender => _senders
+      .firstWhereOrNull((RTCRtpSender item) =>
+          item.track != null && item.track!.kind == 'audio')
+      ?.dtmfSender;
 
   String? get contact => _contact;
 
@@ -628,8 +634,9 @@ class RTCSession extends EventManager implements Owner {
     if (stream != null) {
       switch (sdpSemantics) {
         case 'unified-plan':
-          stream.getTracks().forEach((MediaStreamTrack track) {
-            _connection!.addTrack(track, stream!);
+          stream.getTracks().forEach((MediaStreamTrack track) async {
+            RTCRtpSender sender = await _connection!.addTrack(track, stream!);
+            _senders.add(sender);
           });
           break;
         case 'plan-b':
@@ -2395,8 +2402,9 @@ class RTCSession extends EventManager implements Owner {
     if (stream != null) {
       switch (sdpSemantics) {
         case 'unified-plan':
-          stream.getTracks().forEach((MediaStreamTrack track) {
-            _connection!.addTrack(track, stream!);
+          stream.getTracks().forEach((MediaStreamTrack track) async {
+            RTCRtpSender sender = await _connection!.addTrack(track, stream!);
+            _senders.add(sender);
           });
           break;
         case 'plan-b':

Diff for: lib/src/rtc_session/dtmf.dart

@@ -70,8 +70,8 @@ class DTMF extends EventManager {
     _interToneGap = options['interToneGap'];
 
     if (_mode == DtmfMode.RFC2833) {
-      RTCDTMFSender dtmfSender = _session.dtmfSender;
-      dtmfSender.insertDTMF(_tone!,
+      RTCDTMFSender? dtmfSender = _session.dtmfSender;
+      dtmfSender?.insertDTMF(_tone!,
           duration: _duration!, interToneGap: _interToneGap!);
     } else if (_mode == DtmfMode.INFO) {
       extraHeaders.add('Content-Type: application/dtmf-relay');