### 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<ResourceImporterTexture> import_texture;
		import_texture.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_texture);
		Ref<ResourceImporterLayeredTexture> import_cubemap;
		import_cubemap.instantiate();
		import_cubemap->set_mode(ResourceImporterLayeredTexture::MODE_CUBEMAP);
		ResourceFormatImporter::get_singleton()->add_importer(import_cubemap);
		Ref<ResourceImporterLayeredTexture> import_array;
		import_array.instantiate();
		import_array->set_mode(ResourceImporterLayeredTexture::MODE_2D_ARRAY);
		ResourceFormatImporter::get_singleton()->add_importer(import_array);
		Ref<ResourceImporterLayeredTexture> import_cubemap_array;
		import_cubemap_array.instantiate();
		import_cubemap_array->set_mode(ResourceImporterLayeredTexture::MODE_CUBEMAP_ARRAY);
		ResourceFormatImporter::get_singleton()->add_importer(import_cubemap_array);
		Ref<ResourceImporterLayeredTexture> import_3d;
		import_3d.instantiate();
		import_3d->set_mode(ResourceImporterLayeredTexture::MODE_3D);
		ResourceFormatImporter::get_singleton()->add_importer(import_3d);
		Ref<ResourceImporterImage> import_image;
		import_image.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_image);
		Ref<ResourceImporterTextureAtlas> import_texture_atlas;
		import_texture_atlas.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_texture_atlas);
		Ref<ResourceImporterDynamicFont> import_font_data_dynamic;
		import_font_data_dynamic.instantiate();
		ResourceFormatImporter::get_singleton()->add_importer(import_font_data_dynamic);