how do i fix NullReferenceException: Object reference not set to an instance of an object? AlphabetDataDrawer.OnInspectorGUI () (at Assets/Script/Editor/AlphabetDataDrawer.cs:29)

 using System;
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEditor;
 using UnityEditorInternal;
 
 [CustomEditor(typeof(AlphabetData))]
 [CanEditMultipleObjects]
 [System.Serializable]
 public class AlphabetDataDrawer : Editor
 {
     private ReorderableList AlphabetPlainList;
     private ReorderableList AlphabetNormalList;
     private ReorderableList AlphabetHighlightedList;
     private ReorderableList AlphabetWrongList;
 
     private void OnEnable()
     {
         InitializeReordableList(ref AlphabetPlainList, "AlphabetPlainList", "Alphabet Plain");
         InitializeReordableList(ref AlphabetPlainList, "AlphabetNormalList", "Alphabet Normal");
         InitializeReordableList(ref AlphabetPlainList, "AlphabetHighlightedList", "Alphabet Highlighted");
         InitializeReordableList(ref AlphabetPlainList, "AlphabetWrongList", "Alphabet Wrong");
     }
 
     public override void OnInspectorGUI()
     {
         serializedObject.Update();
         AlphabetPlainList.DoLayoutList();
         AlphabetNormalList.DoLayoutList();
         AlphabetHighlightedList.DoLayoutList();
         AlphabetWrongList.DoLayoutList();
         serializedObject.ApplyModifiedProperties();
 
     }
 
     private void InitializeReordableList(ref ReorderableList list, string propertyName, string listLabel)
     {
         list = new ReorderableList(serializedObject, serializedObject.FindProperty(propertyName),
             true, true, true, true);
 
         list.drawHeaderCallback = (Rect rect) =>
         {
             EditorGUI.LabelField(rect, listLabel);
         };
 
         var l = list;
 
         list.drawElementCallback = (Rect rect, int Index, bool IsActive, bool IsFocused) =>
         { 
             var element = l.serializedProperty.GetArrayElementAtIndex(Index);
             rect.y += 2;
 
             EditorGUI.PropertyField(
                 new Rect (rect.x, rect.y, 60, EditorGUIUtility.singleLineHeight),
                 element.FindPropertyRelative("letter"), GUIContent.none);
 
             EditorGUI.PropertyField(
                 new Rect(rect.x + 70, rect.y, rect.width - 60 - 30, EditorGUIUtility.singleLineHeight),
                 element.FindPropertyRelative("Image"), GUIContent.none);
         };
     }
 }