Multi-editing is not supported with drawDefaultInspector?

I have a simple code:

using UnityEngine;
using UnityEditor;
using System.Collections;

[CustomEditor(typeof(ItemContainer))]
public class ItemContainerEditor : Editor{

    public override void OnInspectorGUI() {
        DrawDefaultInspector();

        EditorGUILayout.HelpBox("The Parent of ItemContainer must have StoreManeger", MessageType.Info);
    }

}

However when I try to multi object editing, it says it is not supported. I have put the script in Assets/Scripts/Editor. What am I doing wrong? ’

Thank you

Your editor doesn’t have the CanEditMultipleObjects attribute.

So you have to do either:

[CustomEditor(typeof(ItemContainer)), CanEditMultipleObjects]
public class ItemContainerEditor : Editor{
// ...

or

[CustomEditor(typeof(ItemContainer))]
[CanEditMultipleObjects]
public class ItemContainerEditor : Editor{
// ...