### 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
Node *old_edited_scene_root = get_editor_data().get_edited_scene_root();
	if (old_edited_scene_root) {
		if (old_edited_scene_root->get_parent() == scene_root) {
			scene_root->remove_child(old_edited_scene_root);
		}
		old_edited_scene_root->disconnect(SNAME("replacing_by"), callable_mp(this, &EditorNode::set_edited_scene));
	}
	get_editor_data().set_edited_scene_root(p_scene);
	if (Object::cast_to<Popup>(p_scene)) {
		Object::cast_to<Popup>(p_scene)->show();
	}
	SceneTreeDock::get_singleton()->set_edited_scene(p_scene);
	if (get_tree()) {
		get_tree()->set_edited_scene_root(p_scene);
	}
	if (p_scene) {
		if (p_scene->get_parent() != scene_root) {
			scene_root->add_child(p_scene, true);
		}
		p_scene->connect(SNAME("replacing_by"), callable_mp(this, &EditorNode::set_edited_scene));
	}
}
int EditorNode::_get_current_main_editor() {