ReorderableList Custom Editor Count -> NullReferenceExeception

Hi I have trouble getting ReorderableList to run in a custom Editor.

I followed this Tutorial: http://va.lent.in/unity-make-your-lists-functional-with-reorderablelist/

But I get an exception:

NullReferenceException: Object reference not set to an instance of an object UnityEditorInternal.ReorderableList.get_count () (at C:/buildslave/unity/build/Editor/Mono/GUI/ReorderableList.cs:371)
UnityEditorInternal.ReorderableList.GetListElementHeight () (at C:/buildslave/unity/build/Editor/Mono/GUI/ReorderableList.cs:424)
UnityEditorInternal.ReorderableList.DoLayoutList () (at C:/buildslave/unity/build/Editor/Mono/GUI/ReorderableList.cs:383)

Here is the relevant code I got so far:

 using UnityEngine;
 using UnityEditor;
 using UnityEditorInternal;
 
 public class MyClass : Monobehaviour
 {
     [SerializeField] 
     private List<object> _entries = new List<object>();
 }
 
 public class MyClassEditor : Editor
 {
     private SerializedProperty _entries;
     private ReorderableList _entryList;
 
     private void OnEnable()
     {
         _entries = serializedObject.FindProperty("_entries");
         _entryList = new ReorderableList(serializedObject, _entries, false, false, false, false);
     }
 
     public override void OnInspectorGUI()
     {
         serializedObject.Update();
 
         _entryList.DoLayoutList();
 
         serializedObject.ApplyModifiedProperties();
     }
 }

The exception occures for the line _entryList.DoLayoutList();.

So I made a brake point in VS and saw that it is always thrown for the count property:

What am I doing wrong?