### 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 (!leading_stylebox.pinned || leading_stylebox.stylebox.is_null()) {
		return;
	}
	
	edited_theme->_freeze_change_propagation();
	List<StringName> names;
	edited_theme->get_stylebox_list(edited_type, &names);
	List<Ref<StyleBox>> styleboxes;
	for (const StringName &E : names) {
		Ref<StyleBox> sb = edited_theme->get_stylebox(E, edited_type);
		
		if (sb == leading_stylebox.stylebox) {
			continue;
		}
		if (sb->get_class() == leading_stylebox.stylebox->get_class()) {
			styleboxes.push_back(sb);
		}
	}
	List<PropertyInfo> props;
	leading_stylebox.stylebox->get_property_list(&props);
	for (const PropertyInfo &E : props) {
		if (!(E.usage & PROPERTY_USAGE_STORAGE)) {
			continue;
		}