problem with serialization(?)

i've runned into this problem. i've a script that call another. i attached the script in the editor by drag and dropping it. the problems comes when i hit play mode, the script attached is lost, but if i stop play mode, it reappears in the editor. i think that is a problem of serialization(?) but i even don't know how to pass it. this is how i call the script.

 public class secondo : MonoBehaviour {
 
     public primo prim;
 
     // Use this for initialization
 
     void Start () {
         
         prim = GetComponent<primo> ();
     }