addChild function
Implementation
Future<void> addChild(WidgetModel model, List<dynamic> arguments) async {
// fml
var xml = elementAt(arguments, 0);
// if index is null, add to end of list.
int? index = toInt(elementAt(arguments, 1));
// silent
bool silent = toBool(elementAt(arguments, 2)) ?? true;
if (xml == null || xml is! String) return;
// append
await _appendXml(model, xml, index, silent);
}