### 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)

ur->add_do_method(*edited_theme, "set_color", p_item_name, p_item_type, Color());
			ur->add_undo_method(*edited_theme, "clear_color", p_item_name, p_item_type);
			break;
		case Theme::DATA_TYPE_CONSTANT:
			ur->add_do_method(*edited_theme, "set_constant", p_item_name, p_item_type, 0);
			ur->add_undo_method(*edited_theme, "clear_constant", p_item_name, p_item_type);
			break;
		case Theme::DATA_TYPE_MAX:
			break; 
	}
	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();
}
void ThemeItemEditorDialog::_remove_theme_type(const String &p_theme_type) {
	Ref<Theme> old_snapshot = edited_theme->duplicate();
	Ref<Theme> new_snapshot = edited_theme->duplicate();
	EditorUndoRedoManager *ur = EditorUndoRedoManager::get_singleton();
	ur->create_action(TTR("Remove Theme Type"));
	new_snapshot->remove_type(p_theme_type);
	ur->add_do_method(*edited_theme, "clear");
	ur->add_do_method(*edited_theme, "merge_with", new_snapshot);
	
	ur->add_undo_method(*edited_theme, "add_type", p_theme_type);
	ur->add_undo_method(*edited_theme, "merge_with", old_snapshot);
	ur->add_do_method(this, "_update_edit_types");
	ur->add_undo_method(this, "_update_edit_types");
	ur->commit_action();
}
void ThemeItemEditorDialog::_remove_data_type_items(Theme::DataType p_data_type, String p_item_type) {