Custom Inspector List Resetting after Play

So, I am making a custom inspector and I have a list that I want to display whenever an element is added. This works fine until I enter play mode or close unity. I have a feeling this has something to do with all the lists that exist here, so I'm leaving all the simple code out. Here are my scripts.

Mission Step:

 [System.Serializable]
 public class MissionStep : ScriptableObject {
 
     // Other Code...
 
     public List<Item> itemsToBeFetched;
     public List<NPC> npcsToBeTalked;
 }

Mission Script:

 [System.Serializable]
 public class Mission : ScriptableObject {

     // Other Code...
 
     [HideInInspector]
     public List<MissionStep> missionStepsList;
 }

Custom Editor:

 public class MissionEditor : Editor {

 Mission mission;
 MissionStep missionStep;

     // Other Code...
 
     public override void OnInspectorGUI()
     {
         base.OnInspectorGUI ();
 
         GUILayout.Space (5);
 
         // Create "Mission Steps" label
         GUILayout.Label ("Mission Steps", EditorStyles.boldLabel);
 
         MissionStepButtons ();
 
         MissionStepHandler ();
 
         if(GUI.changed)
         {
             EditorUtility.SetDirty (mission);
         }
     }
 
     // Other Code....
 
     private void MissionStepHandler()
     {    
         for(int i = 0; i < mission.missionStepsList.Count; i++)
         {
             missionStep = mission.missionStepsList [i];
 
             // Other code...
         }
     }

I know the list doesn't reset because if I do Debug.Log(list.Count) it shows that it has elements. But whenever I try to access missionStep after I click play or re-open unity, I get an error saying that it is not referenced. Through debugging, I found out that I can't access the elements that I created in the list BEFORE I clicked play/exit, but once I create new elements, it accesses them just fine.

Anyone got any idea what is happening? Thank you