运行simple_arms时出现此错误，我不知道这意味着什么，有人可以提供一些帮助吗？ 谢谢！
[INFO] [1532986584.140879, 0.000000]: Spawn status: SpawnModel: Entity pushed to spawn queue, but spawn service timed out waiting for entity to appear in simulation under the name simple_arm
[ERROR] [1532986584.142002, 261.179000]: Spawn service failed. Exiting.
[INFO] [1532986584.420120728, 261.179000000]: Camera Plugin: Using the 'robotNamespace' param: '/'
[urdf_spawner-6] process has died [pid 17667, exit code 1, cmd /opt/ros/melodic/lib/gazebo_ros/spawn_model -urdf -param robot_description -x 0 -y 0 -z 0 -R 0 -P 0 -Y 0 -model simple_arm __name:=urdf_spawner __log:=/home/bill/.ros/log/453351d6-9427-11e8-8a68-b88198114d56/urdf_spawner-6.log].
log file: /home/bill/.ros/log/453351d6-9427-11e8-8a68-b88198114d56/urdf_spawner-6*.log