How to work with free admob events?

I’m having trouble following the user’s reward after watching a video in my app. It happens that the banner is displayed correctly, the video is also uploaded. But when I click on X to close the video, my app closes and I can not figure out what it can be.

    //calls and displays the reward video
    launchVideoReward() {
        let rewardConfig: AdMobFreeRewardVideoConfig = {
            autoShow: true,
            id: 'ca-app-pub-8000726989219599/7547043373'
        };
        this.admob.rewardVideo.config(rewardConfig);
        this.admob.rewardVideo.prepare().then(() => {
            console.log('video executado com sucesso');
        })
    }
    
    //manipulates available events
    ad_events(){
        document.addEventListener('admob.rewardvideo.events.CLOSE', function(event) {
            console.log(event)
            alert('video closed');
            this.admob.rewardvideo.prepare();
        })
    }