QUESTION ABOUT SENSOR on REEMS ARM

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Hello, i am not very familiar with Ros and Gazebo I want to know please, how do I add sensors on a robotic arm REEMS? This is what I have? I have an error : the topic is empty.

<gazebo reference="${name}_${side}_1_link"> <sensor name="arm_contact_sensor" type="contact"> <always_on>true</always_on> <update_rate>30.0</update_rate> <contact> <collision>testCollision</collision> </contact> <plugin name="gazebo_ros_bumper_controller" filename="libgazebo_ros_bumper.so"> <alwayson>true</alwayson> <updaterate>100</updaterate> <bumpertopicname>test_bumper</bumpertopicname> <framename>world</framename> </plugin> </sensor> </gazebo>