unity, SerializedObject.FindProperty不要写在Editor的OnEnable里，要写在OnInspectorGUI里

如果像下面这样写：

using UnityEngine;
using System.Collections;
using UnityEditor;
using System.Collections.Generic;
using UnityEngine.Assertions.Must;
[CustomEditor(typeof(xxxControl))]
public class xxxControlEditor : Editor
{
    SerializedProperty m_a;
    void OnEnable(){

　　m_a=serializedObject.FindProperty ("m_a");
    }
    public override void OnInspectorGUI()
    {

　　  /////DrawDefaultInspector();

serializedObject.Update ();
        EditorGUILayout.PropertyField(m_a,true);
        serializedObject.ApplyModifiedProperties ();
    }

}

则在其它Editor或EditorWindow脚本的中调用

Editor _editor=Editor.CreateEditor(xxxObj.GetComponent<xxxControl>());

就会报如下错误：

NullReferenceException: (null)
UnityEditor.SerializedObject..ctor (UnityEngine.Object[] objs) (at C:/buildslave/unity/build/artifacts/generated/common/editor/SerializedPropertyBindings.gen.cs:72)
UnityEditor.Editor.GetSerializedObjectInternal () (at C:/buildslave/unity/build/artifacts/generated/common/editor/EditorBindings.gen.cs:151)
UnityEditor.Editor.get_serializedObject () (at C:/buildslave/unity/build/artifacts/generated/common/editor/EditorBindings.gen.cs:144)
xxxControlEditor.OnEnable () (at Assets/??/Editor/xxxControlEditor.cs:??)

如果将SerializedObject.FindProperty从OnEnable中改到OnInspectorGUI中，即：

using UnityEngine;
using System.Collections;
using UnityEditor;
using System.Collections.Generic;
using UnityEngine.Assertions.Must;
[CustomEditor(typeof(xxxControl))]
public class xxxControlEditor : Editor
{
    SerializedProperty m_a;
    void OnEnable(){
    }
    public override void OnInspectorGUI()
    {

　　  /////DrawDefaultInspector();

　　  m_a=serializedObject.FindProperty ("m_a");//serializedObject.FindProperty should be called in OnInspectorGUI() instead of OnEnable()

serializedObject.Update ();
        EditorGUILayout.PropertyField(m_a,true);
        serializedObject.ApplyModifiedProperties ();
    }

}

则不会出现上述错误。