Dependent Pages view causes 500 server error when items don't have a subsite

[michalkleiner]

A cross-reference to silverstripe/silverstripe-cms#2554 which could be e.g. dealt with through the pool of hours co-funded by participating agencies.

As the removal of the subsites module from the default recipe is happening, I don't think this should be considered as resolved purely by the subsites module removal from the default recipe installation.