You are trying to subscribe to a topic of (message) type nav_msgs::base_pose_ground_truth. As far as I know, the (named) topic /base_pose_ground_truth is actually of type nav_msgs::Odometry (you can check with rostopic info /base_pose_ground_truth when the node is running).
So replace nav_msgs::base_pose_ground_truth with nav_msgs::Odometry. And don't forget to properly initialize your Subscriber.