Me.ActiveDisc is always NULL while a disc is running, unless you open the ingame combat abilities window (if it wasnt already open). As soon as you open that window, ActiveDisc starts returning the active disc name as expected. I have only confirmed this on test server, not sure about live.
If you need to run a test in game, do the following -
1. make sure the combat abilities window is closed
2. activate any discipline (preferably one that runs more than a few seconds)
3. /echo ${Me.ActiveDisc}
4. open the combat abilities window (ALT+C)
5. /echo ${Me.ActiveDisc}


