リソース定義更新時のエラー

```
InvalidCastException: Specified cast is not valid.
GameCanvas.Editor.GcEditorResourceBuilder.ValidateImages () (at Packages/GameCanvas/Editor/GcEditorResourceBuilder.cs:389)
GameCanvas.Editor.GcEditorResourceBuilder.Build () (at Packages/GameCanvas/Editor/GcEditorResourceBuilder.cs:54)
GameCanvas.Editor.GcEditorMenu.BuildResource () (at Packages/GameCanvas/Editor/GcEditorMenu.cs:55)
```