I've done some test and I've understood why I receive the exception.
I've done this tiny example to let other users avoid this "issue", that is not a real issue.
So,
if the SelectCell is called and the form is not visible, it generates Exception.
If the form is visible, the function is ok.
For users: If you click on the button repeatly when the form is visible, the selected row is no more selected. It's necessary to give the focus on the table to select it. This happens only on Windows, not Windows Mobile.
Question:
how from code I can determine if form is visible?
I have to use a boolean, because I've seen that in Form methods there is only Enabled, not Visible.