### Godot version

4.1 rc3

### System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 30.0.15.1403) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

### Issue description

This™
![image](https://github.com/godotengine/godot/assets/2223172/cb33110f-a5b0-400a-951f-3fa425f05714)

It can still happen, but it's more difficult to trigger.

### Steps to reproduce

1. Create a new scene with a Control node
2. Add Theme to the Control
3. Open Theme Editor
4. Clear the `theme` property in Control
5. Save, with Theme Editor opened
6. Restart

### Minimal reproduction project

N/A
int flg = 0;
	if (EDITOR_GET("filesystem/on_save/compress_binary_resources")) {
		flg |= ResourceSaver::FLAG_COMPRESS;
	}
	flg |= ResourceSaver::FLAG_REPLACE_SUBRESOURCE_PATHS;
	HashSet<String> edited_resources;
	int saved = 0;
	List<Ref<Resource>> cached;
	ResourceCache::get_cached_resources(&cached);
	for (Ref<Resource> res : cached) {
		if (!res->is_edited()) {
			continue;
		}
		String path = res->get_path();
		if (path.begins_with("res:")) {
			int subres_pos = path.find("::");
			if (subres_pos == -1) {
				
				edited_resources.insert(path);
			} else {
				edited_resources.insert(path.substr(0, subres_pos));
			}
		}