Estoy tratando de mostrar la lista con el grupo porfech usando el código a continuación, pero recibo excepcionesNo se puede obtener la propiedad 'forEach' de referencia indefinida o nula alguien puede ayudarme por favor, ¿dónde me equivoqué

home.ts:

this.events = [{
  id: 1,
  category:'camera',
  title: 'First event',
  date: '2017-12-26'
}, {
  id: 2,
  category:'accessories',
  title: 'Second event',
  date: '2017-12-27'
}, {
  id: 3,
  category:'camera',
  title: 'Third event',
  date: '2017-12-26'
}, {
  id: 4,
  category:'accessories',
  title: 'Fouth event',
  date: '2017-12-27'
},{
  id: 5,
  category:'camera',
  title: 'Fifth event',
  date: '2017-12-26'
}]

}

home.html:

<ion-content padding>

    <ion-item-group *ngFor="let group of events | groupBy: 'date'">
        <ion-item-divider color="light">
            {{ group.date }}
        </ion-item-divider>
        <ion-item *ngFor="let event of group.events">{{ event.title }}</ion-item>
    </ion-item-group>

</ion-content>

GroupByDate:

@Pipe({name: 'groupByDate'})
export class GroupByPipeProvider implements PipeTransform {
    transform(collection: Array<any>, property: string = 'date'): Array<any> {
        if(!collection) {
            return null;
        }
        const gc = collection.reduce((previous, current)=> {
            if(!previous[current[property]]) {
                previous[current[property]] = [];
            }
                current.events.forEach(x => previous[current[property]].push(x));
            return previous;
        }, {});
        return Object.keys(gc).map(date => ({ date: date, events: gc[date] }));
        }  
}