Ich bin Benutzer iron-list mit iron-ajax, aber nach einer Antwort von ajax, wenn das Element auf iron-list verschoben wird, was zu einer Null-Ausnahme führt = "Die Eigenschaft '_list' von undefined kann nicht gelesen werden."

<script>

    Polymer({
        is: 'my-home',

        ready: function () {
            this.$._ajax.generateRequest();
        },

        onResponse: function (e) {

            var people = e.detail.response.data;
            people.forEach(function (person) {
                this.$._list.push('items', person);
            });
            // Clear the lower threshold so we can load more data when the user scrolls down again.
            this._scrollTheshold.clearTriggers();
        }

    });

</script>

hier ist mein HTML-Code von Iron-List

 <iron-list id="_list" items="[[data.data]]" as="item">
        <template>

            <div style="margin-top: 0px">

                <!--Card with header image http://placehold.it/350x150/FFC107/000000-->
                <paper-card>

                    <div class="card-content">
                        <h1 class="feed_title">[[item.heading]]</h1>

                        <p class="feed_description">[[item.subheading]]</p>
                    </div>
                    <div class="card-actions">
                        <paper-button class="button-blue">
                            Share
                        </paper-button>
                        <paper-button class="button-blue">
                            Explore
                        </paper-button>
                    </div>
                </paper-card>
            </div>
        </template>
    </iron-list>