GridViewRow Loop Error: Unable to cast object of type System.Web.UI.WebControls.GridViewRow to type System.Data.DataRow

When i am inserting data into gridview then below error is coming 

Unable to cast object of type 'System.Web.UI.WebControls.GridViewRow' to type 'System.Data.DataRow'.

here is my insert query in gridview 

        protected void btnadd_Click(object sender, EventArgs e)
        {
            insertbalgv();
        }

        private void insertbalgv()
        {
            if (DDLitem.SelectedItem != null)
            {
                int codeitem = Convert.ToInt32(DDLitem.SelectedItem.Value.ToString());
                string Descriptionitem = DDLitem.SelectedItem.Text.ToString();

                DataTable dtgridview = ViewState["dt"] as DataTable;
                DataRow dr2 = dtgridview.NewRow();
                bool ifExist = false;

                foreach (DataRow dr in GVbal.Rows)
                {
                    if (dr["BID"].ToString() == txtsearcbn.Text.Trim())
                    {
                        ifExist = true;
                        break;
                    }

                }
                if (!ifExist)
                {
                    dr2["BID"] = txtsearcbn.Text.Trim();
                    dr2["Codeitem"] = codeitem;
                    dr2["Descriptionitem"] = Descriptionitem;
                    dr2["QTY"] = txtqty.Text;

                    dtgridview.Rows.Add(dr2);
                    ViewState["dt"] = dtgridview;
                    GVbal.DataSource = dtgridview;
                    GVbal.DataBind();
                }
                else
                {
                    // this.lbgvck.Visible = true;
                    Response.Write("<script language='javascript'>alert('Bale Already Added');</script>");
                }

                clear();
            }
        }

        private void clear()
        {
            txtsearcbn.Text = "";
        }
    }