fix waitgroup bug which crashes subscriber

server/subscriber.go

@@ -244,9 +244,15 @@ func (s *rpcServer) createSubHandler(sb *subscriber, opts Options) broker.Handle
 				fn = opts.SubWrappers[i-1](fn)
 			}
 
+			if s.wg != nil {
 				s.wg.Add(1)
+			}
+
 			go func() {
+				if s.wg != nil {
 					defer s.wg.Done()
+				}
+
 				results <- fn(ctx, &rpcMessage{
 					topic:       sb.topic,
 					contentType: ct,