4.2 beta - I am using the new 'Ext.grid.plugin.BufferedRenderer'

I had a store proxy creating a column header checkbox item to select all items in a grid - I am trying to get the same code up and running with 4.2 beta - but am having problems with the store reporting no data

store = Ext.create('Ext.data.Store', {
proxy: {
afterRequest: function(req, res) {
Ext.get("selectAll").on('click', function(){
if($("#selectAll").hasClass('x-grid-checkheader')) {
store.proxy.setExtraParam('action',actionVar);
store.each(function(rec){
rec.set('constantrowselected', true);
});
}
store.load();
});
}
}
});

On click of checkbox I get error:
TypeError: data is undefined


Is this a consequence of BufferRenderer change and if so can anyone replicate or advise?