variable method
Implementation
@override
Future<DVCVariable?> variable(String key, dynamic defaultValue) async {
final result = await methodChannel.invokeMethod(
'variable', {"key": key, "defaultValue": defaultValue}) ??
{};
final map = Map<String, dynamic>.from(result);
DVCVariable? variable = map.isNotEmpty ? DVCVariable.fromCodec(map) : null;
return variable;
}