### Godot version

v4.0.beta17.mono.official [c40020513]

### System information

Window 11, Forward+, GTX 1060

### Issue description

Adding two theme variations in a row result to a  crash.

https://user-images.githubusercontent.com/93253123/217037249-1ce1d56d-beb6-4435-83ad-9f8a07fe0e96.mp4



### Steps to reproduce

1. Create a project.
2. Create a theme.
3. Double click to go in the theme editor.
4. Click on manage items buttons.
5. In the add type field type a name to add a type.
6. type another name to add a second type.
7. Crash.

### Minimal reproduction project

[ThemeVariationCrash.zip](https://github.com/godotengine/godot/files/10665863/ThemeVariationCrash.zip)

if (p.usage & PROPERTY_USAGE_RESTART_IF_CHANGED) {
			restart_request_props.insert(p.name);
		}
		PropertyDocInfo doc_info;
		if (use_doc_hints) {
			
			
			StringName classname = doc_name;
			if (!object_class.is_empty()) {
				classname = object_class;
			} else if (Object::cast_to<MultiNodeEdit>(object)) {
				classname = Object::cast_to<MultiNodeEdit>(object)->get_edited_class_name();
			} else if (classname == "") {
				classname = object->get_class_name();
				Resource *res = Object::cast_to<Resource>(object);
				if (res && !res->get_script().is_null()) {
					
					Ref<Script> scr = res->get_script();
					if (scr.is_valid()) {
						Vector<DocData::ClassDoc> docs = scr->get_documentation();
						if (!docs.is_empty()) {
							classname = docs[0].name;
						}
					}
				}
			}
			StringName propname = property_prefix + p.name;
			bool found = false;
			
			HashMap<StringName, HashMap<StringName, PropertyDocInfo>>::Iterator E = doc_info_cache.find(classname);