在动态控制组合框中获取选定项

我已经动态地创建了一个ComboBox，但是我需要从SelectedIndexChanged中选择值。我得到以下错误:

对象引用未设置为对象的实例

private ComboBox TimeIntervel;
int Stimer; 
public void set control()
{
    ComboBox TimeIntervelCmb = new ComboBox();
    TimeIntervelCmb.Location = new Point(50,70);
    TimeIntervelCmb.Name = "ComboBoxTime";
    TimeIntervelCmb.Size = new Size(80, 100);
    TimeIntervelCmb.Items.Add("500");
    TimeIntervelCmb.Items.Add("1000");
    TimeIntervelCmb.Items.Add("2000");
    TimeIntervelCmb.Items.Add("3000");
    TimeIntervelCmb.Items.Add("4000");
    TimeIntervelCmb.Items.Add("5000");
    TimeIntervelCmb.Items.Add("6000");
    TimeIntervelCmb.SelectedIndexChanged += new EventHandler(TimeIntervel_SelectedIndexChanged);
    this.Controls.Add(TimeIntervelCmb);
}
private void TimeIntervel_SelectedIndexChanged(object sender, EventArgs e)
{
    try
    {
        if (TimeIntervel.SelectedItem  != null)// Object reference not set to an instance of an object.-Error
        {
            Stimer = int.Parse(TimeIntervel.SelectedItem.ToString());
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}