Filling list with editor scripts is empty on Play

Hi everybody. Apologies for yet another question about Editor Scripts on this page.

I'm filling a List through an editor script, but it resets on play mode. I read about SerializeField and I guess that I missed something there.

Here are the relevant Scripts:

AssetAudioPair:

 using UnityEngine;
 using System.Collections;
 
 [System.Serializable]
 public class AssetAudioPair {
     public Sprite sprite;
     public AudioClip audioFeedback;
     public string textContent;
 
     public AssetAudioPair(Sprite s, AudioClip a){
         this.sprite = s;
         this.audioFeedback = a;
     }
 
     public AssetAudioPair(){
     }
 }

GameManager:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.EventSystems;
 using System.Collections.Generic;
 using UnityEngine.SceneManagement;
 using System.Linq;
 
 public class GameManager : MonoBehaviour
 {
     [SerializeField]
     public List<AssetAudioPair> imagePairs;
     
     [SerializeField]
     public List<AssetAudioPair> textPairs;
 
 // Lots of other stuff, but these are the relevant fields
 }

GameManagerEditor:

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 using UnityEngine.UI;
 
 [CustomEditor(typeof(GameManager))]
 public class GameManagerEditor : Editor {
     List<AudioClip> feedback;
     public override void OnInspectorGUI () {
         //Called whenever the inspector is drawn for this object.
         DrawDefaultInspector();
         if (GUILayout.Button ("Fill Images")) {
             feedback = new List<AudioClip>();
             var audioPaths = from a in AssetDatabase.GetAllAssetPaths() 
                         where a.Contains("/Audio/new/") 
                         where a.Contains("03_WRONG") 
                         select a;
             foreach(string path in audioPaths){
                 feedback.Add(AssetDatabase.LoadAssetAtPath<AudioClip>(path));
             }
             var imagePaths = from s in AssetDatabase.GetAllAssetPaths ()
                         where s.Contains ("/Textures/GameUI/Images/")
                         select s;
             foreach (string path in imagePaths) {
                 AssetAudioPair x = new AssetAudioPair ((AssetDatabase.LoadAssetAtPath<Sprite> (path)), null);
                 if (!manager.imagePairs.Any(p => p.sprite == x.sprite)) {
                     manager.imagePairs.Add (x);
                 }
             }
             foreach(AssetAudioPair aP in manager.imagePairs){
                 aP.audioFeedback = feedback[Random.Range(0,3)];
             }
             Debug.Log (manager.imagePairs.Count);
         }
     }
 }

Am I serializing the wrong fields? Am I just doing it wrong? What am I missing?

Thanks!