DependentPages view ERROR 500 when item is not a page

[michalkleiner]

When https://github.com/silverstripe/silverstripe-subsites/ module is installed,
a 500 server error is produced when CMS users visit the 'Dependent pages' tab for a page that is linked/referenced in a content block, as it's trying to fetch the subsite details for that blocks while it's not necessarily using the subsite extension.
An example of such setup is a default installation of a CWP 2.5 recipe.