Passing datastore (populated) to a new window via Message - null object ref?

Hi, Powerbuilder 11.5.1

I'm trying to pass contents (say, 100 rows) of one datastore from w_one to w_two by Message.PowerObjectParm. What I do in the first window is set

Message.PowerObjectParm = lds_datastore

and immediately call opening of the second window. In the preopen() of the second window, I try to retrieve this datastore and its contents by

n_ds ds_get_datastore

ds_get_datastore = Message.PowerObjectParm

However, when I call ds_get_datastore.RowCount(), I keep getting null object reference on it. I did a bit of research, and I would understand if the first window was a response window which was CLOSING right when the new window gets opened, as in that case the original datastore gets destroyed. But it's not a response window, and at the time of opening the second window, the first window and its datastore very much still exist, albeit they lose focus, obviously.

So if the first window is not destroyed, and its datastore as well, why am I getting a null object reference when I call a RowCount() on the picked up datastore in the new window?

If this just can't be done, can you suggest alternative ways? I tried to do it by GetFullState() and keeping it in blob, but it seems that I can't pass blob via Message PowerObjectParm somehow. Compiler returns "Incompatible types in assigment: powerobject, blob"?

Thanks for any suggestions