OperationAdvisor - modifiedBy return null

Hello,

I'm currently developping an operation advisor extension (for server-side).
I need to know which IContributor (I only need the handle) has clicked on the "save" button. I'm able to get the source workItem whth all the other attributes I need (owner and custom attributes) but I still not able to know who performed the save action.

Here is the line of code I use for this :

IContributorHandle ch = sourceworkItem.getModifiedBy();

When I debug it, ch is null.

Is it the good way to get this information ?

Best regards,
Sylvain