TXDataWebDataSet 'data undefined' error

Hi
I use TXDataWebDataSet.Insert(); to insert records. First time go fine.. at second time i have :
Cannot set property 'data' of undefined at Object.InternalInitRecord ....

heppen if i close and reopen dataset with persistent fields in it . ..

If i do :
//wdGraf.Close(); // il close da errore : Cannot set property 'data' of undefined at Object.InternalInitRecord ....

else if i do :
While Not wdGraf.IsEmpty() Do wdGraf.Delete(); GO FINE !

Why ??

Please provide a sample project we can compile and reproduce the problem here. Hard to guess what's going on.