### Godot version

v4.0.beta17.mono.official [c40020513]

### System information

Window 11, Forward+, GTX 1060

### Issue description

Adding two theme variations in a row result to a  crash.

https://user-images.githubusercontent.com/93253123/217037249-1ce1d56d-beb6-4435-83ad-9f8a07fe0e96.mp4



### Steps to reproduce

1. Create a project.
2. Create a theme.
3. Double click to go in the theme editor.
4. Click on manage items buttons.
5. In the add type field type a name to add a type.
6. type another name to add a second type.
7. Crash.

### Minimal reproduction project

[ThemeVariationCrash.zip](https://github.com/godotengine/godot/files/10665863/ThemeVariationCrash.zip)

int e_idx = 0;
		for (const StringName &E : theme_types) {
			Ref<Texture2D> item_icon;
			if (E == "") {
				item_icon = get_theme_icon(SNAME("NodeDisabled"), SNAME("EditorIcons"));
			} else {
				item_icon = EditorNode::get_singleton()->get_class_icon(E, "NodeDisabled");
			}
			theme_type_list->add_icon_item(item_icon, E);
			if (E == edited_type) {
				theme_type_list->select(e_idx);
				item_reselected = true;
			}
			e_idx++;
		}
		if (!item_reselected) {
			theme_type_list->select(0);
			_list_type_selected(0);
		} else {
			_update_type_items();
		}
	} else {
		theme_type_list->set_disabled(true);
		theme_type_list->add_item(TTR("None"));
		edited_type = "";
		_update_type_items();
	}
	updating = false;
}
void ThemeTypeEditor::_update_type_list_debounced() {