DataObjectSet is like an array, it contains a list of DataObjects, and it will not have a Title. First you need to iterate on the DataObjectSet by using:
if ( $items ) foreach ( $items as $item ) {
$item->Title;
}
or if you just want first item:
if ( $first = $items->First() ) {
$first->Title;
}
How are you getting your $item? Cause if you get it via get_one you will get proper DataObject and you can query for Title directly. If you do only get, then you will get the set of DataObjects and you need to iterate.
mat