Gets the first sorter from the sorters collection, excluding
any groupers that may be in place
@protected
@return {Ext.util.Sorter} The sorter, null if none exist
function(){
        var sorters = this.sorters.items,
            len = sorters.length,
            i = 0,
            sorter;
            
        for (; i < len; ++i) {
            sorter = sorters[i];
            if (!sorter.isGrouper) {
                return sorter;    
            }
        }
        return null;
    }