Unity Ads cannot show ads: webapp not initialized

I downloaded the latest Unity Ads SDK and followed the instructions to integrate it on my android app.

UnityAds.init(this, "xxxxxxx", null);

The initialization was successful and the log shows that the ad was downloaded.

Initializing Unity Ads version 1508 with gameId xxxxxxx

Requesting Unity Ads ad plan from https://xxxxxxx

Unity Ads initialized with 3 campaigns and 2 zones

Unity Ads cache: File /storage/xxxxxxx/yyyyyyy.mp4 of 1445875 bytes downloaded in 9102ms

I try to show the ad:

if (UnityAds.canShow()) {
    UnityAds.show();
}

Then this error message appears:

Unity Ads cannot show ads: webapp not initialized

What am I missing?

The error is that the IUnityAdsListener (third initialization parameter) is required and cannot be null.

The fix is to add the listener to the init method like below:

UnityAds.init(this, "xxxxxxx", new IUnityAdsListener() {
    @Override
    public void onHide() {
    }

    @Override
    public void onShow() {
    }

    @Override
    public void onVideoStarted() {
    }

    @Override
    public void onVideoCompleted(String s, boolean b) {
    }

    @Override
    public void onFetchCompleted() {
    }

    @Override
    public void onFetchFailed() {
    }
});