Unity5-serializedObject.FindProperty("listname") for ReorderableList doesn't work...

I'm writing CustomEditor for ReorderableList of list.But Unity says NullReferenceException. I find that serializedObject.FindProperty("listname") is null. This is my code.

Element of List

 public class TestClass
 {
     [UnityEngine.SerializeField]
     public TestEnum com,seccom;
 }
 public enum TestEnum
 {
     up,
     down,
     right,
     left,
 }

List

 using System.Collections.Generic;
 using UnityEngine;
 
 public class ListTest : MonoBehaviour
 {
     [SerializeField]
     public List<TestClass> Listtest = new List<TestClass>();
 }

ListEditor

 using UnityEngine;
 using UnityEditor;
 using UnityEditorInternal;
 
 [CanEditMultipleObjects]
 [CustomEditor(typeof(ListTest))]
 public class ListTestEditor : Editor
 {
     private ReorderableList RL;
     private SerializedProperty ListProp;
 
     private void OnEnable()
     {
         //↓doesn't work
         ListProp = serializedObject.FindProperty("Listtest");
         Debug.Log(ListProp);
         //↑return null
         RL = new ReorderableList(serializedObject, ListProp);
         RL.drawElementCallback = (rect, index, isActive, isFocused) =>
         {
             var element = ListProp.GetArrayElementAtIndex(index);
             EditorGUI.PropertyField(rect, element);
         };
     }
     public override void OnInspectorGUI()
     {
         serializedObject.Update();
         RL.DoLayoutList();
         serializedObject.ApplyModifiedProperties();
     }
 }
 
 

Sorry for my poor English.