### 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
parent = (slash_idx >= 0 && parent.length() > 1) ? parent.substr(slash_idx + 1) : parent;
					r_filenames.write[set_idx] = parent + r_filenames[set_idx];
				}
			}
			
			bool can_proceed = false;
			RBSet<int>::Element *E = iset.front();
			while (E) {
				String scene_name = r_filenames[E->get()];
				bool duplicate_found = false;
				for (const int &F : iset) {
					if (E->get() == F) {
						continue;
					}
					String other_scene_name = r_filenames[F];
					if (other_scene_name == scene_name) {
						duplicate_found = true;
						break;
					}
				}
				RBSet<int>::Element *to_erase = duplicate_found ? nullptr : E;
				
				
				
				
				
				String path = p_full_paths[E->get()];
				
				scene_name = scene_name.get_basename();
				if (path.begins_with("res:")) {