### 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)

import_another_theme_dialog->popup_file_dialog();
}
void ThemeItemEditorDialog::_select_another_theme_cbk(const String &p_path) {
	Ref<Theme> loaded_theme = ResourceLoader::load(p_path);
	if (loaded_theme.is_null()) {
		EditorNode::get_singleton()->show_warning(TTR("Invalid file, not a Theme resource."));
		return;
	}
	if (loaded_theme == edited_theme) {
		EditorNode::get_singleton()->show_warning(TTR("Invalid file, same as the edited Theme resource."));
		return;
	}
	import_another_theme_value->set_text(p_path);
	import_other_theme_items->set_base_theme(loaded_theme);
	import_other_theme_items->reset_item_tree();
}
void ThemeItemEditorDialog::_notification(int p_what) {