To me it looks like renderRepeaterLabel is also called for a dummy repeater item (see the call with a NullPage here). That one doesn't have a page associated yet, as it hasn't been saved. It's just used to render the form for adding a new one. So the calls for the existing items succeed, but that last one fails and throws the error.
This can be cured easily by adding a check for $page in the hook:
// Get values of arguments sent to hook (if needed)
$label = $event->arguments(0);
$cnt = $event->arguments(1);
$page = $event->arguments(2);
// Only execute the hook for actual repeater items,
// not for blank placeholders
if($page instanceof NullPage)
return;
// Your code here, perhaps modifying the return value
// just a simple test:
$return = "LABEL " . $page->getForPage()->template();
In a short test, this worked like expected.