### 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 (FileDialog *E : file_dialogs) {
		E->invalidate();
	}
	for (EditorFileDialog *E : editor_file_dialogs) {
		E->invalidate();
	}
	_mark_unsaved_scenes();
	
	String export_error;
	Error err = OK;
	if (!export_defer.preset.is_empty() && !EditorFileSystem::get_singleton()->is_scanning()) {
		String preset_name = export_defer.preset;
		
		
		export_defer.preset = "";
		Ref<EditorExportPreset> export_preset;
		for (int i = 0; i < EditorExport::get_singleton()->get_export_preset_count(); ++i) {
			export_preset = EditorExport::get_singleton()->get_export_preset(i);
			if (export_preset->get_name() == preset_name) {
				break;
			}
			export_preset.unref();