Polymer 1.1: Firefox error for paper-scroll-header-panel

[adrianaioanei]

ReferenceError: event is not defined

For this function, where there's no event declared:

_updateScrollState: function(scrollTop) {
      var deltaScrollTop = scrollTop - this._prevScrollTop;
      var y = Math.max(0, (this.noReveal) ? scrollTop : this._y + deltaScrollTop);

      if (y > this._headerMaxDelta) {
        y = this._headerMaxDelta;
        this._setHeaderState(this.HEADER_STATE_HIDDEN);

      } else if (this.condenses && this._prevScrollTop >= scrollTop && scrollTop >= this._headerMargin) {
        y = Math.max(y, this._headerMargin);
        this._setHeaderState(this.HEADER_STATE_CONDENSED);

      } else if (y === 0) {
        this._setHeaderState(this.HEADER_STATE_EXPANDED);

      } else {
        this._setHeaderState(this.HEADER_STATE_INTERPOLATED);
      }

      if (!event || !this.fixed && y !== this._y) {
        this._transformHeader(y);
      }

      this._prevScrollTop = Math.max(scrollTop, 0);
      this._y = y;
    }