Allow binding of fields to predicates

Steps to reproduce:

• Create a custom Document Type with a complex field in its schema.
• Create a Page Provider.
• Add a Predicate and bind it to the complex field.
  => Error appears (see screenshot)
• Add a Predicate and bind it to one of the complex field's subfields.
  => Error appears (see screenshot)

Expected behavior:

• It should be possible to bind the predicate to the complex field or it's subfield.

Edit:

• Reproduced when binding predicate to system:fulltext.