UTMT throws an exception when trying to save data with new tiles

[Senjay-id]

Describe the bug

When trying to save the data with tiles being added using a script it throws an error
The layer tile isn't even added to the room

Reproducing steps

1. Open the data.win and run the attached script file inside utmt
   script.zip

2. Save the data
   

Setup Details

1. Latest bleeding edge
2. Windows 10 Latest
3. ZERO Sievert

[Miepee]

This is working as expected. See the documentation:

    /// <summary>
    /// The list of layers this room uses. Used in Game Maker Studio: 2 only, as <see cref="Backgrounds"/> and <see cref="Tiles"/> are empty there.
    /// </summary>
    public UndertalePointerList<Layer> Layers { get => _layers; set { _layers = value; UpdateBGColorLayer(); OnPropertyChanged(); } }
    ```

[Senjay-id]

All this time I'm using a wrong function.... welp tyvm for taking a look at it