Only try to close active datagram sessions

src/io/netty/bootstrap/SessionManager.java

@@ -129,9 +129,11 @@ class SessionManager extends ChannelInboundHandlerAdapter {
         // We make a copy of this b/c of concurrent modification, in the event this is closed BEFORE the child-channels are closed
         ArrayList<DatagramSessionChannel> channels = new ArrayList<DatagramSessionChannel>(datagramChannels.values());
         for (DatagramSessionChannel datagramSessionChannel : channels) {
+            if (datagramSessionChannel.isActive()) {
                 datagramSessionChannel.close();
             }
         }
+    }
 
     @Override
     @SuppressWarnings("unchecked")