Custom Page Base Classes that code-behind classes derive from that declare controls as field declarations. Because of the changes to the code-behind model, users who have migrated apps to V2 sometimes see null reference exceptions when accessing these fields on the “grandparent” base-class because the .aspx compiler can’t correctly wire-up the control references to the fields. We’ve now added support for this in V2, and the migration wizard will automatically configure code-behind classes that use custom base classes appropriately for you.