Looked into it again today and fixed it with a work around:

Instead of rowAdded I now listen to rowAdding I invoke another function which does a AJAX post to the server, waits for the new ID to return and updates it in the JSON. Then I push the JSON value in the viewModel datasource:

ui.values.ID = data;
booksModel.books.push(ui.values);

I let the rowAdding function return false, so the default behavior is blocked.

I looked more into this issue, and it seems it gives the error because the new row isn't added to the Knockout datasource. I noticed when I tried to get a cell value after adding a row. So I checked my Knockout model and it only contains the records that were already there, not the one I added just before.

I have got autoCommit enabled on my grid, so why isn't it adding the rows to the knockout model? Do I have to do this manually?

EDIT:

I tried adding the record to the Knockout model like so:

booksModel.books.push(ui.values);

After that I log the model to check if the new row with ID -1 is in there (it is):

console.log(ko.toJS(booksModel.books));

And then I try editting the ID, ui.rowID are set and correct (logged those to):

$("#grid1").igGridUpdating("setCellValue", ui.rowID, "ID", data);

Still giving an error:

...s.responseDataKey]){c=c[this.settings.responseDataKey]}if(e){for(d=0;d<c.length;...