Windows Form Connection string Error: System.Configuration.ConnectionStringSettingsCollection.this[string].get returned null

System.NullReferenceException: 'The object reference is not set to an instance of an object.'

System.Configuration.ConnectionStringSettingsCollection.this[string].get returned null.

?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionStrings>
    <add name = " appltp" connectionString = "Data Source=DESKTOP-CA7TH2J;Initial Catalog=persoDB;Integrated Security=True"/>
  </connectionStrings>
</configuration>

using System.Data.SqlClient;
using System.Configuration;

namespace appltp
{
    public partial class controle : Form
    {
        SqlConnection con = new SqlConnection();
        SqlCommand com = new SqlCommand();
        con.ConnectionString = ConfigurationManager.ConnectionStrings["appltp"].ConnectionString;
        com.CommandText = "insert into controle values ('"+textbox1.Text+"','"+textbox2.Text+"','"+textbox3.Text+"','"+textbox4.Text+"','"+textbox5.Text + "','" + textbox6.Text + "')";
        com.Connection = con;
        con.Open();
        com.ExecuteNonQuery();
        con.Close();
        MessageBox.Show("bien enregistrer");