### 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 idx = tool_menu->get_item_count();
	tool_menu->add_item(p_name, TOOLS_CUSTOM);
	tool_menu->set_item_metadata(idx, p_callback);
}
void EditorNode::add_tool_submenu_item(const String &p_name, PopupMenu *p_submenu) {
	ERR_FAIL_NULL(p_submenu);
	ERR_FAIL_COND(p_submenu->get_parent() != nullptr);
	tool_menu->add_child(p_submenu);
	tool_menu->add_submenu_item(p_name, p_submenu->get_name(), TOOLS_CUSTOM);
}
void EditorNode::remove_tool_menu_item(const String &p_name) {
	for (int i = 0; i < tool_menu->get_item_count(); i++) {
		if (tool_menu->get_item_id(i) != TOOLS_CUSTOM) {
			continue;
		}
		if (tool_menu->get_item_text(i) == p_name) {
			if (tool_menu->get_item_submenu(i) != "") {
				Node *n = tool_menu->get_node(tool_menu->get_item_submenu(i));
				tool_menu->remove_child(n);
				memdelete(n);
			}
			tool_menu->remove_item(i);
			tool_menu->reset_size();
			return;
		}
	}
}
PopupMenu *EditorNode::get_export_as_menu() {