Connection property has not been initialized in C#

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C#

SQL

An error occurred when I update my SQL server compact database (.sdf)
I'm using C# windows form application
This is the error

ExecuteNonQuery: Connection property has not been initialized.

This is my update code
What is the wrong I stuck 3h in here

Thank in advance !

int index = cb1.SelectedIndex; //combo box
int uid=1;
            

            SqlCeConnection con = new SqlCeConnection("Data Source="C:\application\application.sdf;Persist Security Info=False;");
            con.Open();
            SqlCeCommand cm = new SqlCeCommand("UPDATE tbl_user SET u_name=@u_name , u_scl=@u_scl , u_ext=@u_ext WHERE u_id=@u_id");
            cm.Parameters.AddWithValue("@u_id", uid);
            cm.Parameters.AddWithValue("@u_name", txtname.ToString());
            cm.Parameters.AddWithValue("@u_scl", txtscl.ToString());
            cm.Parameters.AddWithValue("@u_ext",index );
           
            try
            {
                int eff = cm.ExecuteNonQuery();
                
                if (eff == 1)
                {
                    con.Close();
                    MessageBox.Show("User Added", "Done");
                }
                else
                {
                    con.Close();
                    MessageBox.Show("not inserted ", "Error");
                }
            }
            catch (SqlCeException ex)
            {
                MessageBox.Show(ex.ToString());
            }