Add asserts on this.viewer.
Add asserts on this.viewer.

avandecreme/openseadragon,openseadragon/openseadragon,openseadragon/openseadragon,rdlester/openseadragon,msalsbery/openseadragon,msalsbery/openseadragon,avandecreme/openseadragon,rdlester/openseadragon,seanich/openseadragon,seanich/openseadragon,LarissaSmith/openseadragon,jmnetus/openseadragon,LarissaSmith/openseadragon,jmnetus/openseadragon