Sprite objectfield flickers in custom inspector editor.

Hi, I got this annoying issue with editor. I got Custom Inspector Editor for simple ScriptableObject. And Sprite Object fields keep flickering, hiding and showing selected sprite and showing blue selection rectangles instead. Anyone has any idea how can this be prevented from happening? Thanks.

ScriptableObject:

 [CreateAssetMenu(fileName = "LevelRoster", menuName = "Picross/LevelRoster", order = 2)]
 public class LevelRoster : ScriptableObject
 {
     [System.Serializable]
     public class ChapterRoster
     {
         public Sprite chapterIcon;
         public Sprite level1Icon;
         public Sprite level2Icon;
         public Sprite level3Icon;
         public Sprite level4Icon;
         public Sprite level5Icon;
     }
 
     public List<ChapterRoster> chapters;
 
     public LevelRoster()
     {
 
     }
 
 }

Editor:

 [CustomEditor(typeof(LevelRoster))]
 public class LevelRosterEditor : Editor
 {
     public override void OnInspectorGUI()
     {
         LevelRoster levels = (LevelRoster)target;
         int index = 1;
         foreach (LevelRoster.ChapterRoster chapter in levels.chapters)
         {
             EditorGUILayout.BeginHorizontal();
             EditorGUILayout.BeginHorizontal();
             chapter.chapterIcon = this.addSpriteField(chapter.chapterIcon,"CH"+index+":");
             EditorGUILayout.EndHorizontal();
             EditorGUILayout.BeginHorizontal();
             chapter.level1Icon = this.addSpriteField(chapter.level1Icon,"1:");
             EditorGUILayout.EndHorizontal();
             EditorGUILayout.BeginHorizontal();
             chapter.level2Icon = this.addSpriteField(chapter.level2Icon,"2:");
             EditorGUILayout.EndHorizontal();
             EditorGUILayout.BeginHorizontal();
             chapter.level3Icon = this.addSpriteField(chapter.level3Icon,"3:");
             EditorGUILayout.EndHorizontal();
             EditorGUILayout.BeginHorizontal();
             chapter.level4Icon = this.addSpriteField(chapter.level4Icon,"4:");
             EditorGUILayout.EndHorizontal();
             EditorGUILayout.BeginHorizontal();
             chapter.level5Icon = this.addSpriteField(chapter.level5Icon,"5:");
             EditorGUILayout.EndHorizontal();
             EditorGUILayout.EndHorizontal();
             index++;
         }
         if (GUI.changed)
             EditorUtility.SetDirty(target);
     }
     private Sprite addSpriteField(Sprite sprite,string label)
     {
         EditorGUILayout.LabelField(label, GUILayout.Width(50));
         return (Sprite)EditorGUILayout.ObjectField(GUIContent.none, sprite, typeof(Sprite), allowSceneObjects: false, options:GUILayout.Width(55));
     }
 }