code: 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});
          }
      }
docs: Call the DropZone constructor using the View's scrolling element only after the grid has been rendered.