please help me fix this error(displaying in listboxes)

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C#

here is the error>
************** Exception Text **************
System.ArgumentOutOfRangeException: InvalidArgument=Value of '0' is not valid for 'SelectedIndex'.
Parameter name: SelectedIndex

my code>

DataSet st = new DataSet();
           string strConnectionString = "Data Source=JAMES-PC\\SQLEXPRESS;Initial Catalog=staff;Integrated Security=True";

           SqlConnection objconnection = new SqlConnection(strConnectionString);
           using (SqlCommand cmd = new SqlCommand("SELECT  [name] FROM [staff1]",
           objconnection))
           {
               using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
               {
                   adapter.Fill(st);
               }
           }

           var empList = st.Tables[0].AsEnumerable().Select(dataRow =>

           dataRow.Field<string>("name")).ToList();

           listBox1.SelectedIndex = 0;
           listBox1.Update();

I dont understand the issue.It worked 10 mins ago but now it doesn't? I am really confused any help would be apreciated