空安全处理

lib/im/SocketClient.dart

@@ -60,6 +60,7 @@ class SocketClient {
       Uint8List data = utf8.encode(jsonEncode({"heartbeat": milliseTime}));
       MsgData msgData = MsgData(from: userId, type: MsgType.TEXT, data: data);
       final proto2 = Proto(3, 1, msgData.writeToBuffer());
+      if(_socket != null)
       _socket.add(proto2.toBytes());
     });
   }