Object reference not set to an instance of an object error on selecteditem of listbox

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C#

ASP.NET

I am using listbox for the first time in my asp.net C# website. I want the user to be able to select multiple items from the listbox and later on the button click event the selected items will be displayed in the label.
I have searched alot on google.. came across many solutions bt non of them worked for me.
I dnt know why m i getting this weird error out of nowhere.

my aspx code:

<asp:ListBox ID="ListBox1" SelectionMode="Multiple" AutoPostBack="true" DataTextField="area" 
            DataValueField="area" runat="server" Height="139px" Width="239px" 
            onselectedindexchanged="ListBox1_SelectedIndexChanged">
        

        <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
       
        You Choosed<br />
        <asp:Label ID="Label1" runat="server">

my code behind aspx.cs code:

protected void Page_Load(object sender, EventArgs e)
   {
       bindlist();
       if (!IsPostBack)
       {
           bindlist();
       }
   }
   protected void bindlist()
   {
       SqlConnection conn = new SqlConnection();
       conn.ConnectionString = "Data Source=SARU-PC\\SQLEXPRESS;Initial Catalog=jd;Integrated Security=True";
       SqlDataAdapter adp = new SqlDataAdapter("select area from area", conn);
       DataSet ds = new DataSet();
       adp.Fill(ds);
       ListBox1.DataSource = ds;
       ListBox1.DataBind();
   }
   protected void Button1_Click(object sender, EventArgs e)
   {
       ListItem item = ListBox1.SelectedItem;
       string a = item.Text;
       Label1.Text = a;
       //for (int i = ListBox1.Items.Count - 1; i >= 0; i--)
       //{
       //    if (ListBox1.Items[i].Selected == true)
       //    {
       //        Label1.Text += ListBox1.Items[i].ToString();
       //    }
       //}
       //ListItem item = ListBox1.SelectedItem;
       //string a = item.ToString();
       //Label1.Text = a;
       //Label1.Text = "";
       //foreach (ListItem li in ListBox1.Items)
       //{
       //    if (li.Selected)
       //    {
       //        Label1.Text += li.ToString() + "<br />";
       //    }
       //}
   }

   protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
   {
       Label1.Text = "";
       string a = ListBox1.SelectedItem.ToString();
       Label1.Text += a + "<br />";
   }

1) I loaded my listbox from database in page load event later i replaced with existing code just in case.
2) I tried getting selected items on button click in label bt later i even tried getting it on selectedindexchanged event. it still doesnt work.
3) have tried diff ways of assigning selected items of listbox in label bt all in vain.

I am getting the"Object reference not set to an instance of an object" error on the line where i assign "listbox1.selecteditem" to a string or to the label itself.

Please help!!!!