ReorderableList Custom Editor Count -> NullReferenceExeception

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

I followed this Tutorial: 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?

Hi never mind _entries was null in the MyClassEditor since object is not serializeable in Unity. I changed it now to use a custom class.