### 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<ConfigFile> cf;
	cf.instantiate();
	if (!DirAccess::exists(addon_path.get_base_dir())) {
		_remove_plugin_from_enabled(addon_path);
		WARN_PRINT("Addon '" + addon_path + "' failed to load. No directory found. Removing from enabled plugins.");
		return;
	}
	Error err = cf->load(addon_path);
	if (err != OK) {
		show_warning(vformat(TTR("Unable to enable addon plugin at: '%s' parsing of config failed."), addon_path));
		return;
	}
	if (!cf->has_section_key("plugin", "script")) {
		show_warning(vformat(TTR("Unable to find script field for addon plugin at: '%s'."), addon_path));
		return;
	}
	String script_path = cf->get_value("plugin", "script");
	Ref<Script> scr; 
	
	if (script_path.length() > 0) {
		script_path = addon_path.get_base_dir().path_join(script_path);
		scr = ResourceLoader::load(script_path, "Script", ResourceFormatLoader::CACHE_MODE_IGNORE);
		if (scr.is_null()) {
			show_warning(vformat(TTR("Unable to load addon script from path: '%s'."), script_path));
			return;
		}
		
		if (scr->get_instance_base_type() == StringName()) {
			if (_initializing_plugins) {