setting default sort order causes error

[mrjones-plip]

on current master branch, if you take the example/index.html which is this:

    var viz = d3.maptable('#vizContainer')
            .csv('basic.csv')
            .map({ path: 'ne_110m_admin_0_countries.json' })
            .filters()
            .table()
            .render();

and change it to this:

    var viz = d3.maptable('#vizContainer')
            .csv('basic.csv')
            .map({ path: 'ne_110m_admin_0_countries.json' })
            .filters()
            .table({
                defaultSorting: {
                    key: "city"
               }
            })
            .render();

there's a JS error in the sortColumn() function: TypeError: d3.event is null - maptable.js 2413 where 2413 is here.