Call the DropZone constructor using the View's scrolling element only after the grid has been rendered.
function(grid) {
        var me = this;

        if (grid.rendered) {
            me.grid = grid;
            grid.getView().on({
                render: function(v) {
                    me.view = v;
                    Ext.ux.dd.CellFieldDropZone.superclass.constructor.call(me, me.view.el);
                },
                single: true
            });
        } else {
            grid.on('render', me.init, me, {single: true});
        }
    }