Optimizely CMS 13 and PageReference is obsolete

While upgrading from Optimizely CMS 12 to Optimizely CMS 13, you may encounter the following warning message:

'PageReference' is obsolete: 'Use ContentReference instead.'

In Optimizely CMS 12 the following property definition will render a property that opens a dialog with the title "Select Page" where you can only select pages.

public virtual PageReference ContactsPage { get; set; }

The dialog, where you can select only pages, looks like this:

If you simply replace PageReference with ContentReference, you may also select images, blocks and forms in addition to pages. That's not always desired. The dialog will have the title "Select Content" and look like this.

To fix this, use the AllowedTypes attribute, like this:

[AllowedTypes(typeof(PageData))]
public virtual ContentReference ContactsPage { get; set; }

Then, the property will look like this, with the added "Only Page types." label:

And the dialog will look exactly like it did with PageReference as the backing type: