### 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)

edited_type = p_type_name;
	bool type_exists = false;
	for (int i = 0; i < theme_type_list->get_item_count(); i++) {
		String type_name = theme_type_list->get_item_text(i);
		if (type_name == edited_type) {
			theme_type_list->select(i);
			type_exists = true;
			break;
		}
	}
	if (type_exists) {
		_update_type_items();
	} else {
		edited_theme->add_icon_type(edited_type);
		edited_theme->add_stylebox_type(edited_type);
		edited_theme->add_font_type(edited_type);
		edited_theme->add_font_size_type(edited_type);
		edited_theme->add_color_type(edited_type);
		edited_theme->add_constant_type(edited_type);
		_update_type_list();
	}
}
bool ThemeTypeEditor::is_stylebox_pinned(Ref<StyleBox> p_stylebox) {