onError method
Implementation
Future<void> onError(JavaScriptMessage message) async {
  if(this.widget.onWebViewError != null) {
    this.widget.onWebViewError!(message);
  }
}
Future<void> onError(JavaScriptMessage message) async {
  if(this.widget.onWebViewError != null) {
    this.widget.onWebViewError!(message);
  }
}