Setting EnableHeaderCheckBox to true causes a NullReferenceException

public partial class RadForm1 : RadForm
{
    private bool flag1 = false;
    private bool flag2 = false;
    private GridViewCheckBoxColumn checkColumn;

    public RadForm1()
    {
        InitializeComponent();
        AddCheckColumn();
        AddButton();

        this.radGridView1.RowCount = 10;
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
        this.radGridView1.MasterTemplate.AllowAddNewRow = false;
    }

    private void AddCheckColumn()
    {
        checkColumn = new GridViewCheckBoxColumn();
        checkColumn.Name = "Select";
        checkColumn.HeaderText = "All";
        checkColumn.EnableHeaderCheckBox = true;
        this.radGridView1.Columns.Insert(0, checkColumn);
        this.radGridView1.HeaderCellToggleStateChanged += RadGridView1_HeaderCellToggleStateChanged;

        this.radGridView1.Columns.Add(new GridViewTextBoxColumn("A"));
        this.radGridView1.Columns.Add(new GridViewTextBoxColumn("B"));
    }

    private void RadGridView1_HeaderCellToggleStateChanged(object sender, GridViewHeaderCellEventArgs e)
    {
        MessageBox.Show($"State：{e.State}");
    }

    private void AddButton()
    {
        Button btn1 = new Button();
        btn1.Text = "Btn1";
        btn1.Click += (o, e) =>
        {
            checkColumn.IsVisible = flag1;
            flag1 = !flag1;
        };
        btn1.Location = new Point(5, 5);
        this.Controls.Add(btn1);

        Button btn2 = new Button();
        btn2.Text = "Btn2";
        btn2.Click += (o, e) =>
        {
            if (flag2)
                radGridView1.Hide();
            else
            {
                radGridView1.Show();
                try
                {
                    radGridView1.ReadOnly = flag2;
                }
                catch { }
            }
            flag2 = !flag2;
        };
        btn2.Location = new Point(120, 5);
        this.Controls.Add(btn2);
    }
}