I Am Trying To Change My Nested Grid-View From Columns To Row And I Am Getting Error In Data-Bind

protected void grdUserMaster_rowdatabound(object sender, GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.DataRow)
            {
                foreach (TableCell cell in e.Row.Cells)
                {
                    DataTable dtdetails=new DataTable();
                    GridView grddetails = e.Row.FindControl("grddetails") as GridView;
                    string userid = (e.Row.Cells[1]).Text;
                    string query="select * from login where userid='"+userid+"'";
                    dtdetails = GetData(query);
                    grddetails.DataSource = ConvertColumnsAsRows(dtdetails);
                    grddetails.DataBind();
                    grddetails.HeaderRow.Visible = false;
                }
            }
        }

public DataTable ConvertColumnsAsRows(DataTable dt)
        {
            DataTable dtnew = new DataTable();
if (dt != null && dt.Rows.Count > 0)
            {
            for (int i = 0; i <= dt.Rows.Count; i++)
            {
                dtnew.Columns.Add(Convert.ToString(i));
            }
            DataRow dr;
            for (int j = 0; j < dt.Columns.Count; j++)
            {
                dr = dtnew.NewRow();
                dr[0] = dt.Columns[j].ToString();
                for (int k = 1; k <= dt.Rows.Count; k++)
                    dr[k] = dt.Rows[k - 1][j];
                dtnew.Rows.Add(dr);
            }
}
            return dtnew;
        }