Undo.RecordObject and Constructor of Serializable Class

I cannot understand why constructor of serializable class is called when doing Undo.RecordObject.
Here is my sample editor-window code.

     public class ConstructorTest: EditorWindow {
 
         [MenuItem( "Window/Test/Constructor" )]
         static void Open() {
             var win = GetWindow<ConstructorTest>();
             win.Show();
 
             Undo.undoRedoPerformed += win.Repaint;
         }
 
         [SerializeField]
         public List<Test1> TestList = new List<Test1>();
         private Vector2 ScrollView;
         private void OnGUI() {
             EditorGUI.BeginChangeCheck();
 
             GUILayout.Label( "Constructor Test" );
             EditorGUILayout.BeginVertical();
             {
                 ScrollView = EditorGUILayout.BeginScrollView( ScrollView );
                 {
                     for ( int i = 0; i < TestList.Count; i++ )
                         TestList[i].OnGUI( i + 1 );
                 }
                 EditorGUILayout.EndScrollView();
             }
             EditorGUILayout.EndVertical();
             GUILayout.FlexibleSpace();
             if( GUILayout.Button( "Add" ) ) {
                 Undo.RecordObject( this, "add obj" );
                 TestList.Add( new Test1() );
             }
 
             if( EditorGUI.EndChangeCheck() )
                 EditorUtility.SetDirty( this );
         }
 
     }
 
     [Serializable]
     public class Test1 {
 
         public Test2 test2 = null;
 
         public Test1() {
             Debug.Log( "Test1 Constructor" );
         }
 
         public void OnGUI( int idx ) {
             EditorGUILayout.BeginVertical( GUI.skin.box );
             {
                 GUILayout.Label( idx.ToString() );
                 if( test2 != null ) {
                     GUILayout.Label( "test2 is not null" );
                 }
             }
             EditorGUILayout.EndVertical();
         }
 
     }
 
     [Serializable]
     public class Test2 {
 
         public Test2() {
             Debug.Log( "Test2 Constructor" );
         }
 
     }

I never write new Test2(), however constructor of Test2 class was called.
When I comment out Undo.RecordObject, constructor of Test2 class was not called.
Why Undo.RecordObject call constructor of Test2 class?
I want to save Test2 Class as null at Test1 Class..
(2017.3.0f2)