Unable to create frames in TileMapLayer

### Tested versions

v4.5.beta7

### System information

Godot v4.5.beta7 - macOS Sequoia (15.4.1) - Multi-window, 1 monitor - Vulkan (Forward+) - integrated Intel(R) UHD Graphics 630 - Intel(R) Core(TM) i7-8700B CPU @ 3.20GHz (12 threads) - 16.00 GiB memory

### Issue description

In a TileMapLayer, I select a Tile, configure the columns but I was not able to ad frames with the "Add frame" button: when clicking this button, nothing happens.

I tried:

- to reload the project (multiple times)
- create another TileMapLayer
- use another tileset asset

But nothing works.

### Steps to reproduce

1. Create a `TileMapLayer`, with a `TileSet` (128x128px in my case)
2. Select the source texture and accept the creation of the Tiles
3. In the **Select** tab, select a tile (1 in the image).
4. Under **Animation**, set the **Columns** (2 in the image)
5. Under **Frames**, click the **Add element** button (3 in the image)

Nothing happens.

### Minimal reproduction project (MRP)

[tilemap_layer_test.zip](https://github.com/user-attachments/files/22115712/tilemap_layer_test.zip)

Open the attached tilemap_layer_test.zip project and reproduce the problem in the TileMapLayer editor using the listed steps. Start by tracing the TileSet animation editor's Frames/Add element action; done means clicking Add element creates a frame reliably after Columns is set.

Written by the indexing model from the issue text.