How to callback specific tf joint from openni_tracker

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hey, I'm trying to write a callback for tf's from the openni_tracker node but i get errors at runtime. To "echo" the data, i launch the tracker with this parameters:

<node pkg="openni_tracker" type="openni_tracker" name="openni_tracker">
<param name="tf_prefix" value="tracker" />
 <param name="camera_frame_id" value="/camera_link" />
</node>

i'm using the following code (excerpt) to get the data:

ros::Subscriber openni_sub = n.subscribe("tf", 1000, openniCallback);
....
void openniCallback(const tf::tfMessage openni_sub) {

    tf::TransformListener listener;
    tf::StampedTransform tf_right_hand;

    ros::NodeHandle n;

    try {
        listener.lookupTransform("/camera_link",
                "/tracker/right_hand_1", ros::Time(0), tf_right_hand);
        x_right_hand = tf_right_hand.getOrigin().x();
                    //same with left_hand and head.
.....

    }
    catch (tf::TransformException ex) {
              ROS_ERROR("%s", ex.what());
        }
    }

My Problem is, that i get the data for right_hand, left_hand and head, but i also get a lot of errors like:

[ERROR] [1372076533.053340930]: Frame id /tracker/right_hand_1 does not exist! Frames (15): Frame /tracker/torso_1 exists with parent /camera_link.

My idea is, that i get a callback, but there is no data to lookup. so:

1. how can i get a callback if there is new data for the right_hand, left_hand and head, not a callback for any tfMessage?
2. is it possible not to use a transformListener.lookupTransform like "openni_sub.getOrigin().x()? - i did not get that one work, too...