### 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)

} break;
		case ITEMS_TREE_REMOVE_ITEM: {
			String item_name = item->get_text(0);
			int data_type = item->get_parent()->get_metadata(0);
			EditorUndoRedoManager *ur = EditorUndoRedoManager::get_singleton();
			ur->create_action(TTR("Remove Theme Item"));
			ur->add_do_method(*edited_theme, "clear_theme_item", (Theme::DataType)data_type, item_name, edited_item_type);
			ur->add_undo_method(*edited_theme, "set_theme_item", (Theme::DataType)data_type, item_name, edited_item_type, edited_theme->get_theme_item((Theme::DataType)data_type, item_name, edited_item_type));
			ur->add_do_method(this, "_update_edit_item_tree", edited_item_type);
			ur->add_undo_method(this, "_update_edit_item_tree", edited_item_type);
			ur->commit_action();
		} break;
		case ITEMS_TREE_REMOVE_DATA_TYPE: {
			int data_type = item->get_metadata(0);
			_remove_data_type_items((Theme::DataType)data_type, edited_item_type);
		} break;
	}
}
void ThemeItemEditorDialog::_add_theme_type(const String &p_new_text) {
	const String new_type = edit_add_type_value->get_text().strip_edges();
	edit_add_type_value->clear();
	EditorUndoRedoManager *ur = EditorUndoRedoManager::get_singleton();
	ur->create_action(TTR("Add Theme Type"));
	ur->add_do_method(*edited_theme, "add_type", new_type);
	ur->add_undo_method(*edited_theme, "remove_type", new_type);
	ur->add_do_method(this, "_update_edit_types");
	ur->add_undo_method(this, "_update_edit_types");
	ur->commit_action();
}
void ThemeItemEditorDialog::_add_theme_item(Theme::DataType p_data_type, String p_item_name, String p_item_type) {