binding combox in windows form application

If you get this error on that line, convert it to:

if(cmbPartyTypeType.SelectedValue != null && cmbPartyTypeType.SelectedValue.ToString() == "1")
...

But, much more convenient way is to use the debugger. Put a breakpoint (with F9) before where you suspect the error. Then run your application. When the debugger hits the breakpoint, go step by step by pressing F10.
By using the debugger, you can find your answer by yourself, and I assure you, you can find it faster compared to waiting for an answer from someone else.

[updated]

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Test_Cmb
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            InitOthers();
        }

        BindingSource bs = new BindingSource();

        private void InitOthers()
        {
            this.bs.DataSource = typeof(MyComboItem);
            this.bs.Add(new MyComboItem()
            {
                DisplayVal = "Select",
                SelectedVal = "0"
            });
            this.bs.Add(new MyComboItem()
            {
                DisplayVal = "Agent",
                SelectedVal = "1"
            });

            this.comboBox1.DisplayMember = "DisplayVal";
            this.comboBox1.ValueMember = "SelectedVal";
            this.comboBox1.SelectedValueChanged += comboBox1_SelectedValueChanged;

            this.comboBox1.DataSource = this.bs;
        }

        void comboBox1_SelectedValueChanged(object sender, EventArgs e)
        {
            this.textBox1.Text = this.comboBox1.SelectedValue == null ?
                "null value" : this.comboBox1.SelectedValue.ToString();

        }
    }

    public class MyComboItem
    {
        public string DisplayVal { get; set; }
        public string SelectedVal { get; set; }
    }
}