sql network server

 Dear all

I got that error when I was trying to connect to a server over IP

Object reference not set to an instance of an object.

 the code for that

string connectionstr = string.Format(@"Data Source=10.192.4.154,1433;Initial Catalog=maindatabase;Integrated Security=True", listBox1.SelectedItem.ToString());
            SqlConnection connection = new SqlConnection();
            connection.ConnectionString = connectionstr;
            connection.Open();
            statusStrip1.Items.Add("Connection Is Open");
            connection.Close();

 Any Ideas?