error "A data item was not found in the container" in the nested grid view

In the main gridview "GridViewTtransmittals" I have the nested gridview with the name "GridViewTranstoCon"
"GridViewTranstoCon" when has data , I have no problem , but when there is not any data for that it returns error "A data item was not found in the container", I like it returns null value or will be unvisible.
The code is like below:
protected void GridViewTtransmittals_RowDataBound(object sender, GridViewRowEventArgs e) { if (e.Row.DataItem == null) return; TransmittaltoConPresentationModel transmittaltoCon = (TransmittaltoConPresentationModel)e.Row.DataItem; GridView g2 = (GridView)e.Row.FindControl("GridViewTranstoCon"); if (transmittaltoCon.TranstoCons != null) { g2.DataSource = transmittaltoCon.TranstoCons; g2.DataBind(); } }  
The code for data source of main gridview  "GridViewTtran ...