How can I get the data in a Grid View control in Web GUI screen?

I am trying unsuccessfully to extract the content of a webGUI screen grid view of from an iframe. For instance, I  access transaction WE02 via WebGUI and I want to get the list of IDOCs (to manage that in a For Each loop). I created the collection for IDOCs in the capture, then in the automation i called the activity to get the element value and paste it to the screen. In the log message no data is displayed. I tried multiple activities but I still fail.

There must have been something I missed. Did some of you already experience this using WebGUI? Is it possible to do this?

Thanks in advance.