VREP IMU plugin: Transform argument source_frame in tf2 frame_ids cannot be empty

edit

Looking at the code for the IMU vrep plugin I don't see it setting the frame_id field. That is most likely the cause here.

Edit:

But how is the function or class property name to set frame_id for given String ?

I'm not sure what you're asking, but to set a proper frame id:

fl.header.frame_id = "<insert some frame name here>"

Edit2: I'm not a VREP user, but the warning shown here sounds like something to pay attention to:

// Quaternion code added
// But It's not from IMU Sensor but from V-Rep simGetObjectQuaternion() Function
// BE CAREFULL When you use it