### 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
Ref<ResourceImporterBMFont> import_font_data_bmfont;
		import_font_data_bmfont.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_font_data_bmfont);
		Ref<ResourceImporterImageFont> import_font_data_image;
		import_font_data_image.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_font_data_image);
		Ref<ResourceImporterCSVTranslation> import_csv_translation;
		import_csv_translation.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_csv_translation);
		Ref<ResourceImporterWAV> import_wav;
		import_wav.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_wav);
		Ref<ResourceImporterOBJ> import_obj;
		import_obj.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_obj);
		Ref<ResourceImporterShaderFile> import_shader_file;
		import_shader_file.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_shader_file);
		Ref<ResourceImporterScene> import_scene;
		import_scene.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_scene);
		Ref<ResourceImporterScene> import_animation;
		import_animation = Ref<ResourceImporterScene>(memnew(ResourceImporterScene(true)));
		ResourceFormatImporter::get_singleton()->add_importer(import_animation);
		{
			Ref<EditorSceneFormatImporterCollada> import_collada;
			import_collada.instantiate();
			ResourceImporterScene::add_importer(import_collada);
			Ref<EditorOBJImporter> import_obj2;
			import_obj2.instantiate();