[iOS] Not working when rootViewController is not FlutterViewController

## System info

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

```objc
+ (void)registerWithRegistrar:(NSObject*)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];
}
```

Start at the registerWithRegistrar entry point shown in the issue and trace how the plugin obtains and uses the application's rootViewController. Reproduce the failure with a root view controller that is not a FlutterViewController; done means the iOS plugin works in that setup without breaking the existing registration path.

索引モデルが issue の本文から書いたものです。