### 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)

if (!p_tree_item->get_meta("_can_be_imported")) {
		return;
	}
	TreeItem *data_type_node = p_tree_item->get_parent();
	if (!data_type_node || data_type_node == import_items_tree->get_root()) {
		return;
	}
	TreeItem *type_node = data_type_node->get_parent();
	if (!type_node || type_node == import_items_tree->get_root()) {
		return;
	}
	ThemeItem ti;
	ti.item_name = p_tree_item->get_text(0);
	ti.data_type = (Theme::DataType)(int)data_type_node->get_metadata(0);
	ti.type_name = type_node->get_text(0);
	if (!selected_items.has(ti)) {
		p_tree_item->set_checked(IMPORT_ITEM, false);
		p_tree_item->set_checked(IMPORT_ITEM_DATA, false);
		return;
	}
	if (selected_items[ti] == SELECT_IMPORT_FULL) {
		p_tree_item->set_checked(IMPORT_ITEM, true);
		p_tree_item->set_checked(IMPORT_ITEM_DATA, true);
	} else if (selected_items[ti] == SELECT_IMPORT_DEFINITION) {
		p_tree_item->set_checked(IMPORT_ITEM, true);
		p_tree_item->set_checked(IMPORT_ITEM_DATA, false);
	}
}
void ThemeItemImportTree::_update_total_selected(Theme::DataType p_data_type) {