### 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
ERR_FAIL_COND_V(!p_object, StringName());
	Ref<Script> scr = p_object->get_script();
	if (scr.is_null() && Object::cast_to<Script>(p_object)) {
		scr = p_object;
	}
	if (scr.is_valid()) {
		Ref<Script> base_scr = scr;
		while (base_scr.is_valid()) {
			StringName name = EditorNode::get_editor_data().script_class_get_name(base_scr->get_path());
			if (name != StringName()) {
				return name;
			}
			
			StringName base = base_scr->get_instance_base_type();
			if (base != StringName() && EditorNode::get_editor_data().get_custom_types().has(base)) {
				const Vector<EditorData::CustomType> &types = EditorNode::get_editor_data().get_custom_types()[base];
				for (int i = 0; i < types.size(); ++i) {
					if (types[i].script == base_scr) {
						return types[i].name;
					}
				}
			}
			base_scr = base_scr->get_base_script();
		}
	}
	return StringName();
}
void EditorNode::_pick_main_scene_custom_action(const String &p_custom_action_name) {