[iOS] Not working when rootViewController is not FlutterViewController

[foxsofter]

System info

Issue occurs on: both Plugin version: all Flutter doctor output: x

+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
    channel = [FlutterMethodChannel
               methodChannelWithName:CHANNEL_NAME
               binaryMessenger:[registrar messenger]];

    UIViewController *viewController = [UIApplication sharedApplication].delegate.window.rootViewController;
    FlutterWebviewPlugin* instance = [[FlutterWebviewPlugin alloc] initWithViewController:viewController];

    [registrar addMethodCallDelegate:instance channel:channel];
}

[foxsofter]

Getting it from the engine when needed is the right solution，because the ViewController of FlutterEngine may change at any time. here