This appears to be a general PHP warning, you’re supplying an incorrect argument in a foreach loop.
$_POST['list_id'] is expected to be an array or any other (iteratable object), when no checkboxes are checked I assume $_POST['list_id'] is completely undefined.
$_POST['list_id'] = isset( $_POST['list_id'] ) ? (array) $_POST['list_id'] : array();
Adding this before you iterate over everything should help you get rid of the warning altogether.
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