Creating a react-native app and trying to ad ads through admob and firebase, but getting an error I couldn't find much on.

Relevant parts of my App.js:

...
import {
  InterstitialAd,
  TestIds,
  AdEventType,
} from '@react-native-firebase/admob';
...
showAds = () => {
    let interstitial = InterstitialAd.createForAdRequest(TestIds.INTERSTITIAL);

    let interstitialListener = interstitial.onAdEvent(type => {
      if (type === AdEventType.LOADED) {
        interstitial.show();
      }
    });
    interstitial.load();

    return () => {
      interstitialListener = null;
    };
  };

onEvent = e => {
    if (e.type === 'game-over') {
      this.setState({
        running: false,
      });
      this.showAds();
};

UPDATE:

Following this guide instead but getting another error.