### 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 (waiting_for_first_scan) {
		waiting_for_first_scan = false;
		OS::get_singleton()->benchmark_end_measure("editor_scan_and_import");
		
		
		RenderingServer::get_singleton()->global_shader_parameters_load_settings(true);
		
		if (!singleton->cmdline_export_mode) {
			EditorResourcePreview::get_singleton()->start();
		}
		_load_editor_layout();
		if (!defer_load_scene.is_empty()) {
			OS::get_singleton()->benchmark_begin_measure("editor_load_scene");
			load_scene(defer_load_scene);
			defer_load_scene = "";
			OS::get_singleton()->benchmark_end_measure("editor_load_scene");
			OS::get_singleton()->benchmark_dump();
		}
	}
}
void EditorNode::_scan_external_changes() {