ShowPaywall method
Implementation
@override
Future<void> ShowPaywall(String jsonString) async {
  try {
    await methodChannel
        .invokeMethod('ShowPaywall', {'jsonString': jsonString});
  } on PlatformException catch (e) {
    print("Failed to call ShowPaywall: '${e.message}'.");
  } on MissingPluginException catch (e) {
    print("Failed to call ShowPaywall: ${e.message}");
  }
  return null;
}