经过一番研究，我发现我的问题与Failed to set remote answer sdp: Called in wrong state: stable问题中的问题相同。这是由于一个RTCPeerConnection只能建立一个对等连接这一事实造成的。因此，我可以通过在每次新客户端希望加入通道时在服务器端创建一个新的RTCPeerConnection来解决此问题。

Future<String> _createRoom() async {
    final db = FirebaseFirestore.instance;
    final roomRef = db.collection('rooms').doc();
    await newPeerConnection(roomRef);

    roomId = roomRef.id;

    // Listening for remote session description below
    roomRef.snapshots().listen((snapshot) async {
      final data = snapshot.data();

      if (data != null && data['answer'] != null) {
        final answer = data['answer'] as Map<String, dynamic>;
        final description = RTCSessionDescription(
          answer['sdp'] as String?,
          answer['type'] as String?,
        );

        await peerConnectionList.last.setRemoteDescription(description);
        await newPeerConnection(roomRef);
      }
    });

    // Listen for remote Ice candidates below
    roomRef.collection('calleeCandidates').snapshots().listen((snapshot) {
      for (final change in snapshot.docChanges) {
        if (change.type == DocumentChangeType.added) {
          final data = change.doc.data();

          peerConnectionList.last.addCandidate(
            RTCIceCandidate(
              data?['candidate'] as String?,
              data?['sdpMid'] as String?,
              data?['sdpMLineIndex'] as int?,
            ),
          );
        }
      }
    });

    return roomId!;
}

Future<void> newPeerConnection(DocumentReference roomRef) async {
    final peerConnection = await createPeerConnection(
      configuration,
      offerSdpConstraints,
    );

    _registerPeerConnectionListeners(peerConnection);

    localStream?.getTracks().forEach((track) {
      peerConnection.addTrack(track, localStream!);
    });

    final offer = await peerConnection.createOffer();
    await peerConnection.setLocalDescription(offer);

    final roomWithOffer = <String, dynamic>{'offer': offer.toMap()};
    await roomRef.set(roomWithOffer);

    peerConnectionList.add(peerConnection);
  }