nlapiGetContext().setSessionObject() launching exception in client script

Hello,
I have two custom records, customrecXXX and customrecYYY.

One of the fields of customrecYYY, let's call it custrecY_X, contains the internal id of a customrecXXX record (i.e. custrecY_X is a "List/Record" field pointing to customrecXXX).

On the "save record" event of the form managing the customrecXXX, I also have to generate a new customrecYYY record entry, containing, in its custrecY_X field, the newly generated id of customrecXXX.
I must generate this customrecYYY on the "save record" event, because I need some informations that I cannot collect on the "after submit" event.

The problem is that, on the "save record" event, I don't have the newly generated id of customrecXXX yet, so I tried to divide the generation of customrecYYY in two different events.