### 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
root_name = EditorNode::adjust_scene_name_casing(root_name);
				file->set_current_path(root_name + "." + extensions.front()->get().to_lower());
			}
			file->popup_file_dialog();
			file->set_title(TTR("Save Scene As..."));
		} break;
		case FILE_SAVE_ALL_SCENES: {
			_save_all_scenes();
		} break;
		case FILE_RUN_SCENE: {
			project_run_bar->play_current_scene();
		} break;
		case FILE_EXPORT_PROJECT: {
			project_export->popup_export();
		} break;
		case FILE_EXTERNAL_OPEN_SCENE: {
			if (unsaved_cache && !p_confirmed) {
				confirmation->set_ok_button_text(TTR("Open"));
				confirmation->set_text(TTR("Current scene not saved. Open anyway?"));
				confirmation->popup_centered();
				break;
			}
			bool oprev = opening_prev;
			Error err = load_scene(external_file);
			if (err == OK && oprev) {
				previous_scenes.pop_back();
				opening_prev = false;
			}
		} break;
		case EDIT_UNDO: {