### 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)

bool sub_inspector = false;
	bool wide_editors = false;
	bool deletable_properties = false;
	float refresh_countdown;
	bool update_tree_pending = false;
	StringName _prop_edited;
	StringName property_selected;
	int property_focusable;
	int update_scroll_request;
	struct PropertyDocInfo {
		String description;
		String path;
	};
	HashMap<StringName, HashMap<StringName, PropertyDocInfo>> doc_info_cache;
	HashMap<StringName, String> class_descr_cache;
	HashSet<StringName> restart_request_props;
	HashMap<ObjectID, int> scroll_cache;
	String property_prefix; 
	String object_class;
	Variant property_clipboard;
	bool restrict_to_basic = false;
	void _edit_set(const String &p_name, const Variant &p_value, bool p_refresh_all, const String &p_changed_field);
	bool _is_main_editor_inspector() const;
	void _property_changed(const String &p_path, const Variant &p_value, const String &p_name = "", bool p_changing = false, bool p_update_all = false);
	void _multiple_properties_changed(Vector<String> p_paths, Array p_values, bool p_changing = false);
	void _property_keyed(const String &p_path, bool p_advance);
	void _property_keyed_with_value(const String &p_path, const Variant &p_value, bool p_advance);
	void _property_deleted(const String &p_path);
	void _property_checked(const String &p_path, bool p_checked);
	void _property_pinned(const String &p_path, bool p_pinned);