### 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
current_menu_option = -1;
	if (accept) {
		accept->set_ok_button_text(p_title);
		accept->set_text(p_text);
		EditorInterface::get_singleton()->popup_dialog_centered(accept);
	}
}
void EditorNode::show_save_accept(const String &p_text, const String &p_title) {
	current_menu_option = -1;
	if (save_accept) {
		save_accept->set_ok_button_text(p_title);
		save_accept->set_text(p_text);
		EditorInterface::get_singleton()->popup_dialog_centered(save_accept);
	}
}
void EditorNode::show_warning(const String &p_text, const String &p_title) {
	if (warning) {
		warning->set_text(p_text);
		warning->set_title(p_title);
		EditorInterface::get_singleton()->popup_dialog_centered(warning);
	} else {
		WARN_PRINT(p_title + " " + p_text);
	}
}
void EditorNode::_copy_warning(const String &p_str) {