system.nullreferenceexception:object reference not set to an instance at program.main

[nebrom]

i am using C# desktop application and ms sql server database.
i can run my project in the local pc when i run it from my project or from the .exe file in the debug folder. it runs fine.
but when i copy the .exe file from debug or release folder and put it in other folder even in the same pc.. it show me error

system.nullreferenceexception:object reference not set to an instance of an object.
at Form1..ctor()
 at program.main()

where form1 is the main form of my application.

please help me

 public partial class Form1 : Form
    {
        
         SqlConnection dbcon = new SqlConnection(ConfigurationManager.ConnectionStrings["rentaldbcon"].ConnectionString);
      
        public Form1()
        {
          
            InitializeComponent();
         
           
        }

static class Program
    {
        /// Program.cs
        [STAThread]
        static void Main()
        {
           
            try
            {
                Application.EnableVisualStyles();
                Application.SetCompatibleTextRenderingDefault(false);
                Application.Run(new Form1());
            }
            catch (Exception ex)
            {
              MessageBox.Show(ex.ToString());
            }
        }

help me please

[jmcilhinney]

Are you copying the config file with the EXE? That connection string your loading is stored in the config file so, if there's not config file, there's no connection string.