### 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
for (int i = p_start; i < editor_data.get_edited_scene_count(); i++) {
		if (!editor_data.get_edited_scene_root(i)) {
			continue;
		}
		bool unsaved = EditorUndoRedoManager::get_singleton()->is_history_unsaved(editor_data.get_scene_history_id(i));
		if (unsaved) {
			String scene_filename = editor_data.get_edited_scene_root(i)->get_scene_file_path();
			if (p_valid_filename && scene_filename.is_empty()) {
				continue;
			}
			return i;
		}
	}
	return -1;
}
void EditorNode::_exit_editor(int p_exit_code) {