Xiaojun,
The warning you're seeing means that something is going wrong in the laser filters that are being applied to the tilt laser. There are a number of different filters applied in the pr2_navigation_perception package and you'll probably want to check the output of each to see what's going on. Off the top of my head:
tilt_scan->tilt_scan_shadow_filtered (shadow filtering)
tilt_scan_shadow_filtered->tilt_scan_filtered (filtering based on the URDF of the robot, in its default configuration will not work for a robot that is not the PR2, this is probably where things are going wrong)
tilt_scan_filtered->ground_object_cloud (filtering out of obstacles that lie on the ground)
Also, using the pr2_2dnav application on a robot that is not the PR2 probably wont work, though the components of the application can give you insight on how to structure a similar one. I'd suggest taking a look at how the application works and creating a similar one for your robot that has the necessary changes. There's a tutorial on this here: http://www.ros.org/wiki/navigation/Tutorials/RobotSetup
Hope this helps,
Eitan