c# VisualStudio error System.NullReferenceException when use string equals

Question

C# WindowsForms App in Visual Studio 2015 community. I create combobox item with 2 values When i create button and whant add compare condition.

client its string = text selected in combobox
If client = text1 value - launch app1
If client = text2 value - launch app2

But when i use equal function i get error.

    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;
    using System.Diagnostics;

namespace WindowsFormsApplication3
public partial class Form1 : Form
string client;

public Form1()

InitializeComponent();  

public void comboBox1_SelectedIndexChanged(object sender, EventArgs e)

comboBox1.DisplayMember.ToString();
string client = comboBox1.Text;

public void button1_Click_1(object sender, EventArgs e)

if (client.Equals("notepad")) 
Process.Start("notepad.exe");

But when i launch app i get System.NullReferenceException error.