OK, this happens because (in theory) whenever you view the list of group pages there is a "Copy page" button next to any which are copyable. However, blocktype/groupviews/groupviews.json.php doesn't import pieforms, meaning it fails when trying to create the button. So the simple fix is to require pieforms in that page.
OK, this happens because (in theory) whenever you view the list of group pages there is a "Copy page" button next to any which are copyable. However, blocktype/ groupviews/ groupviews. json.php doesn't import pieforms, meaning it fails when trying to create the button. So the simple fix is to require pieforms in that page.