Launch a modalPopup when selecting a row in a gridview

[Don Juan]

Hi all
I have a Gridview and a modalPopupExtender, I linked the event:
Gridview1_SelectedIndexCHanged (and changing) in order to show the
ModalPopup.
This appears but only when I click twice, e.g. I need to click in "select"
field and nothing appears, so I click again and it works, really I don't
know where the trouble is.

Thanks for your comments.