get updateComplete() {
        return this._updatePromise;
    }
Returns a Promise that resolves when the element has completed updating.
The Promise value is a boolean that is `true` if the element completed the
update without triggering another update. The Promise result is `false` if
a property was set inside `updated()`. This getter can be implemented to
await additional state. For example, it is sometimes useful to await a
rendered element before fulfilling this Promise. To do this, first await
`super.updateComplete` then any subsequent state.

@returns {Promise} The Promise returns a boolean that indicates if the
update resolved without triggering another update.