Do you have your object ZAG_IDOC defined in your container? When you open the container from SWI1, can you see this object and does look like it is initial? You must have an object instance in your workflow/task container, if you want to find the object in the table.
I don't think you even need to instantiate the object. Just create the event with SAP_WAPI_CREATE_EVENT.
Regards,
Karri