### 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)

String error;
	if (p_name == "") {
		error = TTR("Metadata name can't be empty.");
	} else if (!p_name.is_valid_identifier()) {
		error = TTR("Metadata name must be a valid identifier.");
	} else if (object->has_meta(p_name)) {
		error = vformat(TTR("Metadata with name \"%s\" already exists."), p_name);
	} else if (p_name[0] == '_') {
		error = TTR("Names starting with _ are reserved for editor-only metadata.");
	}
	if (error != "") {
		add_meta_error->add_theme_color_override("font_color", get_theme_color(SNAME("error_color"), SNAME("Editor")));
		add_meta_error->set_text(error);
		add_meta_dialog->get_ok_button()->set_disabled(true);
	} else {
		add_meta_error->add_theme_color_override("font_color", get_theme_color(SNAME("success_color"), SNAME("Editor")));
		add_meta_error->set_text(TTR("Metadata name is valid."));
		add_meta_dialog->get_ok_button()->set_disabled(false);
	}
}
void EditorInspector::_show_add_meta_dialog() {