when i update Error Shows As Object Reference Not Set To an instance of object.please help me i send my below code as follows;

Question

 protected void ResultGridView_RowUpdating(object sender, GridViewUpdateEventArgs e)

    {

  TextBox txtFName = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtFName");

        TextBox txtLName = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtLName");

        TextBox txtCity = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtCity");

        TextBox txtState = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtState");

        TextBox txtCountry = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtCountry");

        TextBox txtDescription = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtDescription");

 

 cmd.Connection = con;

 cmd.CommandText = "Update Vendors Set VendorFName = '" + txtFName.Text + "',VendorLName = '" + txtLName.Text + "',VendorCity = '" + txtCity.Text + "',VendorState = '" + txtState.Text + "',VendorCountry = '" + txtCountry.Text + "',VendorDescription = '" + txtDescription.Text + "' where VendorId = " + ResultGridView.DataKeys[e.RowIndex].Values[0].ToString() + "";

        con.Open();

        cmd.ExecuteNonQuery();

        ResultGridView.EditIndex = -1;

        FillVendorGrid();

        con.Close();

 

From the above what is the error please correct it.