### 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 (object != nullptr) {
		_edit_request_change(object, String());
	}
}
void EditorInspector::_vscroll_changed(double p_offset) {
	if (update_scroll_request >= 0) { 
		return;
	}
	if (object) {
		scroll_cache[object->get_instance_id()] = p_offset;
	}
}
void EditorInspector::set_property_prefix(const String &p_prefix) {
	property_prefix = p_prefix;
}
String EditorInspector::get_property_prefix() const {
	return property_prefix;
}
void EditorInspector::set_object_class(const String &p_class) {
	object_class = p_class;
}
String EditorInspector::get_object_class() const {