I have a error with Admob banner in Flutter.

This is my error.

I/flutter (25842): AdWidget requires Ad.load to be called before AdWidget is inserted into the tree

And this is my code about Admob banner

class GoogleAdMob {
  static BannerAd loadBannerAd() {
    BannerAd banner = BannerAd(
      adUnitId: globals.admobAdUnitIdBanner!,
      size: AdSize.banner,
      request: const AdRequest(),
      listener: BannerAdListener(
        onAdLoaded: (
          Ad ad,
        ) =>
        onAdFailedToLoad: (
          Ad ad,
          LoadAdError error,
        ) {
          ad.dispose();
        onAdOpened: (
          Ad ad,
        ) =>
        onAdClosed: (
          Ad ad,
        ) =>
        onAdImpression: (
          Ad ad,
        ) =>
      ),
    );
    return banner;
  }

  static Container showBannerAd(
    BannerAd banner,
  ) {
    final Container adContainer = Container(
      alignment: Alignment.center,
      width: banner.size.width.toDouble(),
      height: banner.size.height.toDouble(),
      child: AdWidget(
        ad: banner,
      ),
    );

    return adContainer;
  }
}

What’s matter about the error?