### 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
DEV_ASSERT(Thread::get_caller_id() == Thread::get_main_id());
	singleton->load_errors->add_image(singleton->gui_base->get_theme_icon(SNAME("Warning"), SNAME("EditorIcons")));
	singleton->load_errors->add_text(p_warning + "\n");
	EditorInterface::get_singleton()->popup_dialog_centered_ratio(singleton->load_error_dialog, 0.5);
}
bool EditorNode::_find_scene_in_use(Node *p_node, const String &p_path) const {
	if (p_node->get_scene_file_path() == p_path) {
		return true;
	}
	for (int i = 0; i < p_node->get_child_count(); i++) {
		if (_find_scene_in_use(p_node->get_child(i), p_path)) {
			return true;
		}
	}
	return false;
}
bool EditorNode::is_scene_in_use(const String &p_path) {
	Node *es = get_edited_scene();
	if (es) {
		return _find_scene_in_use(es, p_path);
	}
	return false;
}
OS::ProcessID EditorNode::has_child_process(OS::ProcessID p_pid) const {
	return project_run_bar->has_child_process(p_pid);
}
void EditorNode::stop_child_process(OS::ProcessID p_pid) {