Issue in Live Class 51 (Fusing Odometry and IMU)

I am trying to do the live ROSject #51. I am following the instructions step by step and a problem comes up when I open RViz and the specific configuration, odom_config given in the class. The RobotModel has a major error stating that there is no transform between the base_link and the respective wheel_joints, therefore the whole robot cannot be displayed in RViz. What seems to be the problem there? Is there a launch file I need to execute before opening RViz? I am trying to do this class as it is very helpful towards learning about the robot_localization package. Thank you for your help in advance!