The <p:dataTable>
is also a NamingContainer
. Include its ID as well.
<p:blockUI ... trigger="wfTable:editWatchfolderButtonId">
The row index is only present in client side, not in server side, so that's not relevant.
Update: just tested it locally, it indeed fixes the exception, but it didn't trigger the block UI at all (PrimeFaces 3.5). Look like a bug in PrimeFaces.
In the meanwhile, your best bet is to manually trigger it as suggested by Aksel, but then somewhat different:
<p:commandLink ... onclick="bui.show()" oncomplete="bui.hide()">
...
<p:blockUI widgetVar="bui" />