ビルドシーンに一覧からシーン選択できるアトリビュート。登録シーンが消えても手動で変更するまで保持します

SceneAttribute.cs

using System.Linq;
using UnityEngine;
#if UNITY_EDITOR
using UnityEditor;
#endif

class SceneAttribute : PropertyAttribute
{
}

#if UNITY_EDITOR
[CustomPropertyDrawer(typeof(SceneAttribute))]
class SceneDrawer : PropertyDrawer
{
    static readonly string DummySceneName = "__Deleted__";
    
    bool _init;
    int _index;
    string[] _sceneNames;

    public SceneDrawer()
    {
        var scenes = EditorBuildSettings.scenes
            .Where(scene => scene.enabled)
            .Select(scene => System.IO.Path.GetFileNameWithoutExtension(scene.path));

        if (scenes.Count() == 0)
        {
            scenes = scenes.Concat(new string[] { DummySceneName });
        }

        _sceneNames = scenes.ToArray();
    }

    void GetCurrentSelectIndex(SerializedProperty property, ref int index)
    {
        string sceneName = property.stringValue;
        for (int i = 0; i < _sceneNames.Length; i++)
        {
            if (sceneName == _sceneNames[i])
            {
                _index = i;
                return;
            }
        }
        _index = _sceneNames.Length - 1;
    }

    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        if (property.propertyType == SerializedPropertyType.String)
        {
            property.serializedObject.Update();

            if (!_init)
            {
                GetCurrentSelectIndex(property, ref _index);
                _init = true;
            }

            _index = EditorGUI.Popup(position, property.displayName, _index, _sceneNames);
            if (_sceneNames[_index] != DummySceneName)
            {
                property.stringValue = _sceneNames[_index];
            }

            property.serializedObject.ApplyModifiedProperties();
            return;
        }
        EditorGUI.PropertyField(position, property, label, true);
    }
}
#endif