### 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
EditorNode::ExecuteThreadArgs *eta = (EditorNode::ExecuteThreadArgs *)p_ud;
	Error err = OS::get_singleton()->execute(eta->path, eta->args, &eta->output, &eta->exitcode, true, &eta->execute_output_mutex);
	print_verbose("Thread exit status: " + itos(eta->exitcode));
	if (err != OK) {
		eta->exitcode = err;
	}
	eta->done.set();
}
int EditorNode::execute_and_show_output(const String &p_title, const String &p_path, const List<String> &p_arguments, bool p_close_on_ok, bool p_close_on_errors) {
	if (execute_output_dialog) {
		execute_output_dialog->set_title(p_title);
		execute_output_dialog->get_ok_button()->set_disabled(true);
		execute_outputs->clear();
		execute_outputs->set_scroll_follow(true);
		EditorInterface::get_singleton()->popup_dialog_centered_ratio(execute_output_dialog);
	}
	ExecuteThreadArgs eta;
	eta.path = p_path;
	eta.args = p_arguments;
	eta.exitcode = 255;
	int prev_len = 0;
	eta.execute_output_thread.start(_execute_thread, &eta);