[r2r_common] Do not panic in get_wanted_messages() if AMENT_PREFIX_PATH is not set

r2r_common/src/lib.rs

@@ -148,13 +148,16 @@ pub fn get_wanted_messages() -> Vec<RosMsg> {
         get_ros_msgs_files(&dirs)
     } else {
         // Else we look for all msgs we can find using the ament prefix path.
-        let ament_prefix_var = env::var("AMENT_PREFIX_PATH").expect("Source your ROS!");
+        if let Ok(ament_prefix_var) = env::var("AMENT_PREFIX_PATH") {
             let paths = ament_prefix_var
                 .split(':')
                 .map(Path::new)
                 .collect::<Vec<_>>();
 
             get_ros_msgs(&paths)
+        } else {
+            vec![]
+        }
     };
 
     let msgs = parse_msgs(&msgs);