How to clear the datagridview rows when I selected nothing in the combobox

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C#

I have Bankname combobox,and accountno textbox,accounttype combobox and one datagridview.
Now i select bankname from combobox accountno and accounttype showing respective fields and showing the related data in datagridview.After selecting nothing from bankname combobox accountno and accounttype clearing fine.but datagriddata is not clearing.I have searched in google and tried different ways getting Cannot clear the list error.Please Anybody could help me please.
thanks in advance.

What I have tried:

<blockquote class="quote"><div class="op">Quote:</div>  
    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            DataTable dt = new DataTable();
            string abc = cmbbankname.SelectedValue.ToString();
            if (abc != "[select, select]" && abc != "select")
            {
                SqlConnection con = new SqlConnection(connection);
                con.Open();
                SqlCommand cmd = new SqlCommand("SELECT [bank_Id],[bank_Accountno],bank_Accountype FROM bankdetails where bank_Id=@bank_Id", con);
                cmd.Parameters.AddWithValue("@bank_Id", cmbbankname.SelectedValue.ToString());
                SqlDataAdapter sda = new SqlDataAdapter(cmd);
                sda.Fill(dt);
                if (dt.Rows.Count > 0)
                {
                    txtAcntno.Text = dt.Rows[0]["bank_Accountno"].ToString();
                    cmbAcntype.Text = dt.Rows[0]["bank_Accountype"].ToString().Trim();
                    LoadDataGridView();
                }
            }
            else if (cmbbankname.SelectedIndex == 0)
            {
                txtAcntno.Text = "";
                cmbAcntype.SelectedIndex = 0;
                dgvwChqs.Rows.Clear();


            }
        }




         private void LoadDataGridView()
         {
            SqlConnection con = new SqlConnection(connection);
            SqlCommand cmd = new SqlCommand("SELECT bank_Id,row_number() over(order by 
   [bank_Id]) as SlNo,[bank_Chqstartno],bank_Chqlvscount,[bank_Chqendno],bank_Stat FROM 
   [dbo].bankcheques where bank_Id=@bank_Id order by SlNo desc ", con);
            cmd.Parameters.AddWithValue("@bank_Id", cmbbankname.SelectedValue.ToString());
            try
            {
                con.Open();
                SqlDataAdapter da = new SqlDataAdapter();
                da.SelectCommand = cmd;
                DataTable dt = new DataTable();
                da.Fill(dt);
                dgvwChqs.DataSource = dt;
                con.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        </blockquote>