@Override
  public void toData(DataOutput out,
      SerializationContext context) throws IOException {
    context.getSerializer().writeObject(coordinator, out);
    context.getSerializer().writeObject(senderId, out);
    StaticSerialization.writeByteArray(coordinatorPublicKey, out);
    StaticSerialization.writeString(rejectionMessage, out);
    out.writeBoolean(isShortForm);
    out.writeBoolean(fromView);
    out.writeBoolean(networkPartitionDetectionEnabled);
    out.writeBoolean(usePreferredCoordinators);
    context.getSerializer().writeObject(view, out);
    GMSUtil.writeSetOfMemberIDs(registrants, out, context);
  }
When the response comes from a locator via TcpClient this will return the locators member ID.
If the locator hasn't yet joined this may be null.
geode-membership/src/main/java/org/apache/geode/distributed/internal/membership/gms/locator/FindCoordinatorResponse.java