I have a problem about the package gazebo_ros_demos.
When I use roslaunch rrbot_description rrbot_rviz.launch then roslaunch rrbot_control rrbot_control.launch, it shows the warn that is [WARN] [1597821207.287022]: Controller Spawner couldn't find the expected controller_manager ROS interface .
When I use roslaunch rrbot_gazebo rrbot_world.launch then roslaunch rrbot_control rrbot_control.launch, it's ok.
And I compare the result of rosservice list , it shows the former one doesn't have controller_manager . That's why ?