Why do my GameObjects (prefabs) disappear from my ScriptableObject when re-opening Unity?

I have the following class

 using UnityEngine;
 using System; //for serializable
 using System.Collections;
 using System.Collections.Generic;//for list
 
 [Serializable]
 public class Scenario : ScriptableObject
 {
 
     [Serializable]        //player structure, tells player what to do
     public class FootballPlayer
     {
         public GameObject AIPlayer;
 
         public bool waitForBall;
         public bool goToNext;
         public bool takeTheShot;
 
         public Vector3 pos;
     }
 
     [Serializable]        //part structure, contains everything for each part of the scenario
     public class Part
     {
         public List<FootballPlayer> players;
         
         public string playerHint;    //used for giving the player instructions
         
         public GameObject playerToSwitch;    //the player that has the ball for the next scenario
     }
 
     public List<Part> parts;    //allows flexibility when choosing number of parts
 
     public GameObject humanPlayer;    //which player will the human control
 
     public string objectName = "Scenario";    //used for file name
     }

This works perfectly with my custom editor until I close Unity. When the object is created, all the GameObjects (prefabs), strings and bools are fine but when Unity is re-opened, the strings and bools are still there but the GameObjects are all set to empty.