非パブリックなSerializeFieldを持つクラスを継承した際の不具合を修正

[takuma-komatsu]

下記のようなコードで発生する不具合に暫定対処しました。

using System;
using System.Collections.Generic;
using UnityEngine;

[CreateAssetMenu]
public class ErrorSample : ErrorSampleBase
{
}

public abstract class ErrorSampleBase : ScriptableObject
{
    [Serializable]
    public class /* or struct */ Nested
    {
        public string Value;
    }

    // Valueがインスペクタに表示されない
    [SerializeField] private Nested _nested;

    // 「+」ボタンを押すと例外（※下記）が発生する
    [SerializeField] private List<Nested> _nestedList;

    // intやstringだと問題なく表示される
    [SerializeField] private string _stringValue;
    [SerializeField] private List<string> _stringList;
}

発生する例外

NullReferenceException: Object reference not set to an instance of an object
Alchemy.Editor.SerializedPropertyExtensions.GetElementAtOrDefault (System.Object arrayOrListObj, System.Int32 index) (at Assets/Alchemy/Editor/Internal/SerializedPropertyExtensions.cs:204)
Alchemy.Editor.SerializedPropertyExtensions.GetNestedObject[T] (System.String path, System.Object obj, System.Boolean includeAllBases) (at Assets/Alchemy/Editor/Internal/SerializedPropertyExtensions.cs:168)
Alchemy.Editor.SerializedPropertyExtensions.GetValue[T] (UnityEditor.SerializedProperty property) (at Assets/Alchemy/Editor/Internal/SerializedPropertyExtensions.cs:55)
Alchemy.Editor.Elements.AlchemyPropertyField..ctor (UnityEditor.SerializedProperty property, System.Type type, System.Boolean isArrayElement) (at Assets/Alchemy/Editor/Elements/AlchemyPropertyField.cs:62)
Alchemy.Editor.Elements.PropertyListView+<>c__DisplayClass0_1.<.ctor>b__0 (UnityEngine.UIElements.VisualElement element, System.Int32 index) (at Assets/Alchemy/Editor/Elements/PropertyListView.cs:27)

[nuskey8]

PRをマージしました。ありがとうございます！