I have been looking into this with the debugger and it seems that
is called before
This makes me think that the rich:columnGroup is being elaborated at least ONCE (three times in my case) BEFORE rich:dataTable.
I know you get this strange kind of stuff happening in JSF (it really makes JSF unresponsive as often routines are called multiple times).
I should actually learn more about JSF so I can no process some things when its not the right phase!
Anyhow, I hope this helps you analyse what is going on with rich:columnGroup (and possibly update the docs if its a problem that you cant get round).
I have looked at this again and still cant see why it is happening.
I can confirm it is happening however.
That's because dataTable processes (decodes, validates, renders, etc.) column headers prior to rows that contain data.
Thanks for the reply.
Perhaps this is worth a comment in the doco for others like me who are left wondering about all the NPE's