### 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
if (p_resource->get_path().is_resource_file()) {
			file->set_current_file(p_resource->get_path().get_file());
		} else {
			if (extensions.size()) {
				String resource_name_snake_case = p_resource->get_class().to_snake_case();
				file->set_current_file("new_" + resource_name_snake_case + "." + preferred.front()->get().to_lower());
			} else {
				file->set_current_file(String());
			}
		}
	} else if (!p_resource->get_path().is_empty()) {
		file->set_current_path(p_resource->get_path());
		if (extensions.size()) {
			String ext = p_resource->get_path().get_extension().to_lower();
			if (extensions.find(ext) == nullptr) {
				file->set_current_path(p_resource->get_path().replacen("." + ext, "." + extensions.front()->get()));
			}
		}
	} else if (preferred.size()) {
		String existing;
		if (extensions.size()) {
			String resource_name_snake_case = p_resource->get_class().to_snake_case();
			existing = "new_" + resource_name_snake_case + "." + preferred.front()->get().to_lower();
		}
		file->set_current_path(existing);
	}
	file->set_title(TTR("Save Resource As..."));
	file->popup_file_dialog();
}
void EditorNode::_menu_option(int p_option) {