### 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
_save_scene_with_preview(p_file);
				project_run_bar->play_main_scene((bool)pick_main_scene->get_meta("from_native", false));
			}
		} break;
		case FILE_EXPORT_MESH_LIBRARY: {
			Ref<MeshLibrary> ml;
			if (file_export_lib_merge->is_pressed() && FileAccess::exists(p_file)) {
				ml = ResourceLoader::load(p_file, "MeshLibrary");
				if (ml.is_null()) {
					show_accept(TTR("Can't load MeshLibrary for merging!"), TTR("OK"));
					return;
				}
			}
			if (ml.is_null()) {
				ml = Ref<MeshLibrary>(memnew(MeshLibrary));
			}
			MeshLibraryEditor::update_library_file(editor_data.get_edited_scene_root(), ml, true, file_export_lib_apply_xforms->is_pressed());
			Error err = ResourceSaver::save(ml, p_file);
			if (err) {
				show_accept(TTR("Error saving MeshLibrary!"), TTR("OK"));
				return;
			} else if (ResourceCache::has(p_file)) {
				
				ResourceLoader::load(p_file)->reload_from_file();
			}
		} break;
		case RESOURCE_SAVE:
		case RESOURCE_SAVE_AS: {
			ERR_FAIL_COND(saving_resource.is_null());
			save_resource_in_path(saving_resource, p_file);