### 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)

TreeItem *child_item = p_root_item->get_first_child();
	while (child_item) {
		child_item->set_checked(IMPORT_ITEM, true);
		if (p_select_with_data) {
			child_item->set_checked(IMPORT_ITEM_DATA, true);
		}
		_store_selected_item(child_item);
		_select_all_subitems(child_item, p_select_with_data);
		child_item = child_item->get_next();
	}
}
void ThemeItemImportTree::_deselect_all_subitems(TreeItem *p_root_item, bool p_deselect_completely) {
	TreeItem *child_item = p_root_item->get_first_child();
	while (child_item) {
		child_item->set_checked(IMPORT_ITEM_DATA, false);
		if (p_deselect_completely) {
			child_item->set_checked(IMPORT_ITEM, false);
		}
		_store_selected_item(child_item);
		_deselect_all_subitems(child_item, p_deselect_completely);
		child_item = child_item->get_next();
	}
}
void ThemeItemImportTree::_select_all_items_pressed() {